Dockerfile reference
Docker can build images automatically by reading the instructions from a Dockerfile. A Dockerfile is a text document that contains all the commands a user could call on the command line to assemble an image. This page describes the commands you can use in a Dockerfile.
Overview
The Dockerfile supports the following instructions:
Instruction | Description |
---|---|
ADD | Add local or remote files and directories. |
ARG | Use build-time variables. |
CMD | Specify default commands. |
COPY | Copy files and directories. |
ENTRYPOINT | Specify default executable. |
ENV | Set environment variables. |
EXPOSE | Describe which ports your application is listening on. |
FROM | Create a new build stage from a base image. |
HEALTHCHECK | Check a container's health on startup. |
LABEL | Add metadata to an image. |
MAINTAINER | Specify the author of an image. |
ONBUILD | Specify instructions for when the image is used in a build. |
RUN | Execute build commands. |
SHELL | Set the default shell of an image. |
STOPSIGNAL | Specify the system call signal for exiting a container. |
USER | Set user and group ID. |
VOLUME | Create volume mounts. |
WORKDIR | Change working directory. |
Format
Here is the format of the Dockerfile:
# Comment
INSTRUCTION arguments
The instruction is not case-sensitive. However, convention is for them to be UPPERCASE to distinguish them from arguments more easily.
Docker runs instructions in a Dockerfile in order. A Dockerfile must
begin with a FROM
instruction. This may be after
parser
directives,
comments, and globally scoped
ARGs. The FROM
instruction specifies the
parent
image from which you are
building. FROM
may only be preceded by one or more ARG
instructions, which
declare arguments that are used in FROM
lines in the Dockerfile.
BuildKit treats lines that begin with #
as a comment, unless the line is
a valid
parser directive. A #
marker anywhere
else in a line is treated as an argument. This allows statements like:
# Comment
RUN echo 'we are running some # of cool things'
Comment lines are removed before the Dockerfile instructions are executed.
The comment in the following example is removed before the shell executes
the echo
command.
RUN echo hello \
# comment
world
The following examples is equivalent.
RUN echo hello \
world
Comments don't support line continuation characters.
Note
Note on whitespace
For backward compatibility, leading whitespace before comments (
#
) and instructions (such asRUN
) are ignored, but discouraged. Leading whitespace is not preserved in these cases, and the following examples are therefore equivalent:# this is a comment-line RUN echo hello RUN echo world
# this is a comment-line RUN echo hello RUN echo world
Whitespace in instruction arguments, however, isn't ignored. The following example prints
hello world
with leading whitespace as specified:RUN echo "\ hello\ world"
Parser directives
Parser directives are optional, and affect the way in which subsequent lines
in a Dockerfile are handled. Parser directives don't add layers to the build,
and don't show up as build steps. Parser directives are written as a
special type of comment in the form # directive=value
. A single directive
may only be used once.
The following parser directives are supported:
Once a comment, empty line or builder instruction has been processed, BuildKit no longer looks for parser directives. Instead it treats anything formatted as a parser directive as a comment and doesn't attempt to validate if it might be a parser directive. Therefore, all parser directives must be at the top of a Dockerfile.
Parser directive keys, such as syntax
or check
, aren't case-sensitive, but
they're lowercase by convention. Values for a directive are case-sensitive and
must be written in the appropriate case for the directive. For example,
#check=skip=jsonargsrecommended
is invalid because the check name must use
Pascal case, not lowercase. It's also conventional to include a blank line
following any parser directives. Line continuation characters aren't supported
in parser directives.
Due to these rules, the following examples are all invalid:
Invalid due to line continuation:
# direc \
tive=value
Invalid due to appearing twice:
# directive=value1
# directive=value2
FROM ImageName
Treated as a comment because it appears after a builder instruction:
FROM ImageName
# directive=value
Treated as a comment because it appears after a comment that isn't a parser directive:
# About my dockerfile
# directive=value
FROM ImageName
The following unknowndirective
is treated as a comment because it isn't
recognized. The known syntax
directive is treated as a comment because it
appears after a comment that isn't a parser directive.
# unknowndirective=value
# syntax=value
Non line-breaking whitespace is permitted in a parser directive. Hence, the following lines are all treated identically:
#directive=value
# directive =value
# directive= value
# directive = value
# dIrEcTiVe=value
The following parser directives are supported:
syntax
escape
syntax
Use the syntax
parser directive to declare the Dockerfile syntax version to
use for the build. If unspecified, BuildKit uses a bundled version of the
Dockerfile frontend. Declaring a syntax version lets you automatically use the
latest Dockerfile version without having to upgrade BuildKit or Docker Engine,
or even use a custom Dockerfile implementation.
Most users will want to set this parser directive to docker/dockerfile:1
,
which causes BuildKit to pull the latest stable version of the Dockerfile
syntax before the build.
# syntax=docker/dockerfile:1
For more information about how the parser directive works, see Custom Dockerfile syntax.
escape
# escape=\
Or
# escape=`
The escape
directive sets the character used to escape characters in a
Dockerfile. If not specified, the default escape character is \
.
The escape character is used both to escape characters in a line, and to
escape a newline. This allows a Dockerfile instruction to
span multiple lines. Note that regardless of whether the escape
parser
directive is included in a Dockerfile, escaping is not performed in
a RUN
command, except at the end of a line.
Setting the escape character to `
is especially useful on
Windows
, where \
is the directory path separator. `
is consistent
with
Windows PowerShell.
Consider the following example which would fail in a non-obvious way on
Windows. The second \
at the end of the second line would be interpreted as an
escape for the newline, instead of a target of the escape from the first \
.
Similarly, the \
at the end of the third line would, assuming it was actually
handled as an instruction, cause it be treated as a line continuation. The result
of this Dockerfile is that second and third lines are considered a single
instruction:
FROM microsoft/nanoserver
COPY testfile.txt c:\\
RUN dir c:\
Results in:
PS E:\myproject> docker build -t cmd .
Sending build context to Docker daemon 3.072 kB
Step 1/2 : FROM microsoft/nanoserver
---> 22738ff49c6d
Step 2/2 : COPY testfile.txt c:\RUN dir c:
GetFileAttributesEx c:RUN: The system cannot find the file specified.
PS E:\myproject>
One solution to the above would be to use /
as the target of both the COPY
instruction, and dir
. However, this syntax is, at best, confusing as it is not
natural for paths on Windows, and at worst, error prone as not all commands on
Windows support /
as the path separator.
By adding the escape
parser directive, the following Dockerfile succeeds as
expected with the use of natural platform semantics for file paths on Windows:
# escape=`
FROM microsoft/nanoserver
COPY testfile.txt c:\
RUN dir c:\
Results in:
PS E:\myproject> docker build -t succeeds --no-cache=true .
Sending build context to Docker daemon 3.072 kB
Step 1/3 : FROM microsoft/nanoserver
---> 22738ff49c6d
Step 2/3 : COPY testfile.txt c:\
---> 96655de338de
Removing intermediate container 4db9acbb1682
Step 3/3 : RUN dir c:\
---> Running in a2c157f842f5
Volume in drive C has no label.
Volume Serial Number is 7E6D-E0F7
Directory of c:\
10/05/2016 05:04 PM 1,894 License.txt
10/05/2016 02:22 PM <DIR> Program Files
10/05/2016 02:14 PM <DIR> Program Files (x86)
10/28/2016 11:18 AM 62 testfile.txt
10/28/2016 11:20 AM <DIR> Users
10/28/2016 11:20 AM <DIR> Windows
2 File(s) 1,956 bytes
4 Dir(s) 21,259,096,064 bytes free
---> 01c7f3bef04f
Removing intermediate container a2c157f842f5
Successfully built 01c7f3bef04f
PS E:\myproject>
check
# check=skip=<checks|all>
# check=error=<boolean>
The check
directive is used to configure how
build checks
are evaluated. By default, all checks are run, and failures are treated as
warnings.
You can disable specific checks using #check=skip=<check-name>
. To specify
multiple checks to skip, separate them with a comma:
# check=skip=JSONArgsRecommended,StageNameCasing
To disable all checks, use #check=skip=all
.
By default, builds with failing build checks exit with a zero status code
despite warnings. To make the build fail on warnings, set #check=error=true
.
# check=error=true
To combine both the skip
and error
options, use a semi-colon to separate
them:
# check=skip=JSONArgsRecommended;error=true
To see all available checks, see the
build checks reference.
Note that the checks available depend on the Dockerfile syntax version. To make
sure you're getting the most up-to-date checks, use the
syntax
directive to specify the Dockerfile syntax version to the latest stable
version.
Environment replacement
Environment variables (declared with
the ENV
statement) can also be
used in certain instructions as variables to be interpreted by the
Dockerfile. Escapes are also handled for including variable-like syntax
into a statement literally.
Environment variables are notated in the Dockerfile either with
$variable_name
or ${variable_name}
. They are treated equivalently and the
brace syntax is typically used to address issues with variable names with no
whitespace, like ${foo}_bar
.
The ${variable_name}
syntax also supports a few of the standard bash
modifiers as specified below:
${variable:-word}
indicates that ifvariable
is set then the result will be that value. Ifvariable
is not set thenword
will be the result.${variable:+word}
indicates that ifvariable
is set thenword
will be the result, otherwise the result is the empty string.
The following variable replacements are supported in a pre-release version of
Dockerfile syntax, when using the # syntax=docker/dockerfile-upstream:master
syntax
directive in your Dockerfile:
${variable#pattern}
removes the shortest match ofpattern
fromvariable
, seeking from the start of the string.str=foobarbaz echo ${str#f*b} # arbaz
${variable##pattern}
removes the longest match ofpattern
fromvariable
, seeking from the start of the string.str=foobarbaz echo ${str##f*b} # az
${variable%pattern}
removes the shortest match ofpattern
fromvariable
, seeking backwards from the end of the string.string=foobarbaz echo ${string%b*} # foobar
${variable%%pattern}
removes the longest match ofpattern
fromvariable
, seeking backwards from the end of the string.string=foobarbaz echo ${string%%b*} # foo
${variable/pattern/replacement}
replace the first occurrence ofpattern
invariable
withreplacement
string=foobarbaz echo ${string/ba/fo} # fooforbaz
${variable//pattern/replacement}
replaces all occurrences ofpattern
invariable
withreplacement
string=foobarbaz echo ${string//ba/fo} # fooforfoz
In all cases, word
can be any string, including additional environment
variables.
pattern
is a glob pattern where ?
matches any single character
and *
any number of characters (including zero). To match literal ?
and *
,
use a backslash escape: \?
and \*
.
You can escape whole variable names by adding a \
before the variable: \$foo
or \${foo}
,
for example, will translate to $foo
and ${foo}
literals respectively.
Example (parsed representation is displayed after the #
):
FROM busybox
ENV FOO=/bar
WORKDIR ${FOO} # WORKDIR /bar
ADD . $FOO # ADD . /bar
COPY \$FOO /quux # COPY $FOO /quux
Environment variables are supported by the following list of instructions in the Dockerfile:
ADD
COPY
ENV
EXPOSE
FROM
LABEL
STOPSIGNAL
USER
VOLUME
WORKDIR
ONBUILD
(when combined with one of the supported instructions above)
You can also use environment variables with RUN
, CMD
, and ENTRYPOINT
instructions, but in those cases the variable substitution is handled by the
command shell, not the builder. Note that instructions using the exec form
don't invoke a command shell automatically. See
Variable
substitution.
Environment variable substitution use the same value for each variable throughout the entire instruction. Changing the value of a variable only takes effect in subsequent instructions. Consider the following example:
ENV abc=hello
ENV abc=bye def=$abc
ENV ghi=$abc
- The value of
def
becomeshello
- The value of
ghi
becomesbye
.dockerignore file
You can use .dockerignore
file to exclude files and directories from the
build context. For more information, see
.dockerignore file.
Shell and exec form
The RUN
, CMD
, and ENTRYPOINT
instructions all have two possible forms:
INSTRUCTION ["executable","param1","param2"]
(exec form)INSTRUCTION command param1 param2
(shell form)
The exec form makes it possible to avoid shell string munging, and to invoke commands using a specific command shell, or any other executable. It uses a JSON array syntax, where each element in the array is a command, flag, or argument.
The shell form is more relaxed, and emphasizes ease of use, flexibility, and readability. The shell form automatically uses a command shell, whereas the exec form does not.
Exec form
The exec form is parsed as a JSON array, which means that you must use double-quotes (") around words, not single-quotes (').
ENTRYPOINT ["/bin/bash", "-c", "echo hello"]
The exec form is best used to specify an ENTRYPOINT
instruction, combined
with CMD
for setting default arguments that can be overridden at runtime. For
more information, see
ENTRYPOINT.
Variable substitution
Using the exec form doesn't automatically invoke a command shell. This means
that normal shell processing, such as variable substitution, doesn't happen.
For example, RUN [ "echo", "$HOME" ]
won't handle variable substitution for
$HOME
.
If you want shell processing then either use the shell form or execute a shell
directly with the exec form, for example: RUN [ "sh", "-c", "echo $HOME" ]
.
When using the exec form and executing a shell directly, as in the case for the
shell form, it's the shell that's doing the environment variable substitution,
not the builder.
Backslashes
In exec form, you must escape backslashes. This is particularly relevant on Windows where the backslash is the path separator. The following line would otherwise be treated as shell form due to not being valid JSON, and fail in an unexpected way:
RUN ["c:\windows\system32\tasklist.exe"]
The correct syntax for this example is:
RUN ["c:\\windows\\system32\\tasklist.exe"]
Shell form
Unlike the exec form, instructions using the shell form always use a command shell. The shell form doesn't use the JSON array format, instead it's a regular string. The shell form string lets you escape newlines using the escape character (backslash by default) to continue a single instruction onto the next line. This makes it easier to use with longer commands, because it lets you split them up into multiple lines. For example, consider these two lines:
RUN source $HOME/.bashrc && \
echo $HOME
They're equivalent to the following line:
RUN source $HOME/.bashrc && echo $HOME
You can also use heredocs with the shell form to break up supported commands.
RUN <<EOF
source $HOME/.bashrc && \
echo $HOME
EOF
For more information about heredocs, see Here-documents.
Use a different shell
You can change the default shell using the SHELL
command. For example:
SHELL ["/bin/bash", "-c"]
RUN echo hello
For more information, see SHELL.
FROM
FROM [--platform=<platform>] <image> [AS <name>]
Or
FROM [--platform=<platform>] <image>[:<tag>] [AS <name>]
Or
FROM [--platform=<platform>] <image>[@<digest>] [AS <name>]
The FROM
instruction initializes a new build stage and sets the
base image for subsequent
instructions. As such, a valid Dockerfile must start with a FROM
instruction.
The image can be any valid image.
ARG
is the only instruction that may precedeFROM
in the Dockerfile. See Understand how ARG and FROM interact.FROM
can appear multiple times within a single Dockerfile to create multiple images or use one build stage as a dependency for another. Simply make a note of the last image ID output by the commit before each newFROM
instruction. EachFROM
instruction clears any state created by previous instructions.- Optionally a name can be given to a new build stage by adding
AS name
to theFROM
instruction. The name can be used in subsequentFROM <name>
,COPY --from=<name>
, andRUN --mount=type=bind,from=<name>
instructions to refer to the image built in this stage. - The
tag
ordigest
values are optional. If you omit either of them, the builder assumes alatest
tag by default. The builder returns an error if it can't find thetag
value.
The optional --platform
flag can be used to specify the platform of the image
in case FROM
references a multi-platform image. For example, linux/amd64
,
linux/arm64
, or windows/amd64
. By default, the target platform of the build
request is used. Global build arguments can be used in the value of this flag,
for example
automatic platform ARGs
allow you to force a stage to native build platform (--platform=$BUILDPLATFORM
),
and use it to cross-compile to the target platform inside the stage.
Understand how ARG and FROM interact
FROM
instructions support variables that are declared by any ARG
instructions that occur before the first FROM
.
ARG CODE_VERSION=latest
FROM base:${CODE_VERSION}
CMD /code/run-app
FROM extras:${CODE_VERSION}
CMD /code/run-extras
An ARG
declared before a FROM
is outside of a build stage, so it
can't be used in any instruction after a FROM
. To use the default value of
an ARG
declared before the first FROM
use an ARG
instruction without
a value inside of a build stage:
ARG VERSION=latest
FROM busybox:$VERSION
ARG VERSION
RUN echo $VERSION > image_version
RUN
The RUN
instruction will execute any commands to create a new layer on top of
the current image. The added layer is used in the next step in the Dockerfile.
RUN
has two forms:
# Shell form:
RUN [OPTIONS] <command> ...
# Exec form:
RUN [OPTIONS] [ "<command>", ... ]
For more information about the differences between these two forms, see shell or exec forms.
The shell form is most commonly used, and lets you break up longer instructions into multiple lines, either using newline escapes, or with heredocs:
RUN <<EOF
apt-get update
apt-get install -y curl
EOF
The available [OPTIONS]
for the RUN
instruction are:
Option | Minimum Dockerfile version |
---|---|
--mount | 1.2 |
--network | 1.3 |
--security | 1.1.2-labs |
Cache invalidation for RUN instructions
The cache for RUN
instructions isn't invalidated automatically during
the next build. The cache for an instruction like
RUN apt-get dist-upgrade -y
will be reused during the next build. The
cache for RUN
instructions can be invalidated by using the --no-cache
flag, for example docker build --no-cache
.
See the Dockerfile Best Practices guide for more information.
The cache for RUN
instructions can be invalidated by
ADD
and
COPY
instructions.
RUN --mount
RUN --mount=[type=<TYPE>][,option=<value>[,option=<value>]...]
RUN --mount
allows you to create filesystem mounts that the build can access.
This can be used to:
- Create bind mount to the host filesystem or other build stages
- Access build secrets or ssh-agent sockets
- Use a persistent package management cache to speed up your build
The supported mount types are:
Type | Description |
---|---|
bind (default) | Bind-mount context directories (read-only). |
cache | Mount a temporary directory to cache directories for compilers and package managers. |
tmpfs | Mount a tmpfs in the build container. |
secret | Allow the build container to access secure files such as private keys without baking them into the image or build cache. |
ssh | Allow the build container to access SSH keys via SSH agents, with support for passphrases. |
RUN --mount=type=bind
This mount type allows binding files or directories to the build container. A bind mount is read-only by default.
Option | Description |
---|---|
target , dst , destination 1 | Mount path. |
source | Source path in the from . Defaults to the root of the from . |
from | Build stage, context, or image name for the root of the source. Defaults to the build context. |
rw ,readwrite | Allow writes on the mount. Written data will be discarded. |
RUN --mount=type=cache
This mount type allows the build container to cache directories for compilers and package managers.
Option | Description |
---|---|
id | Optional ID to identify separate/different caches. Defaults to value of target . |
target , dst , destination 1 | Mount path. |
ro ,readonly | Read-only if set. |
sharing | One of shared , private , or locked . Defaults to shared . A shared cache mount can be used concurrently by multiple writers. private creates a new mount if there are multiple writers. locked pauses the second writer until the first one releases the mount. |
from | Build stage, context, or image name to use as a base of the cache mount. Defaults to empty directory. |
source | Subpath in the from to mount. Defaults to the root of the from . |
mode | File mode for new cache directory in octal. Default 0755 . |
uid | User ID for new cache directory. Default 0 . |
gid | Group ID for new cache directory. Default 0 . |
Contents of the cache directories persists between builder invocations without invalidating the instruction cache. Cache mounts should only be used for better performance. Your build should work with any contents of the cache directory as another build may overwrite the files or GC may clean it if more storage space is needed.
Example: cache Go packages
# syntax=docker/dockerfile:1
FROM golang
RUN --mount=type=cache,target=/root/.cache/go-build \
go build ...
Example: cache apt packages
# syntax=docker/dockerfile:1
FROM ubuntu
RUN rm -f /etc/apt/apt.conf.d/docker-clean; echo 'Binary::apt::APT::Keep-Downloaded-Packages "true";' > /etc/apt/apt.conf.d/keep-cache
RUN --mount=type=cache,target=/var/cache/apt,sharing=locked \
--mount=type=cache,target=/var/lib/apt,sharing=locked \
apt update && apt-get --no-install-recommends install -y gcc
Apt needs exclusive access to its data, so the caches use the option
sharing=locked
, which will make sure multiple parallel builds using
the same cache mount will wait for each other and not access the same
cache files at the same time. You could also use sharing=private
if
you prefer to have each build create another cache directory in this
case.
RUN --mount=type=tmpfs
This mount type allows mounting tmpfs
in the build container.
Option | Description |
---|---|
target , dst , destination 1 | Mount path. |
size | Specify an upper limit on the size of the filesystem. |
RUN --mount=type=secret
This mount type allows the build container to access secret values, such as tokens or private keys, without baking them into the image.
By default, the secret is mounted as a file. You can also mount the secret as
an environment variable by setting the env
option.
Option | Description |
---|---|
id | ID of the secret. Defaults to basename of the target path. |
target , dst , destination | Mount the secret to the specified path. Defaults to /run/secrets/ + id if unset and if env is also unset. |
env | Mount the secret to an environment variable instead of a file, or both. (since Dockerfile v1.10.0) |
required | If set to true , the instruction errors out when the secret is unavailable. Defaults to false . |
mode | File mode for secret file in octal. Default 0400 . |
uid | User ID for secret file. Default 0 . |
gid | Group ID for secret file. Default 0 . |
Example: access to S3
# syntax=docker/dockerfile:1
FROM python:3
RUN pip install awscli
RUN --mount=type=secret,id=aws,target=/root/.aws/credentials \
aws s3 cp s3://... ...
$ docker buildx build --secret id=aws,src=$HOME/.aws/credentials .
Example: Mount as environment variable
The following example takes the secret API_KEY
and mounts it as an
environment variable with the same name.
# syntax=docker/dockerfile:1
FROM alpine
RUN --mount=type=secret,id=API_KEY,env=API_KEY \
some-command --token-from-env API_KEY
Assuming that the API_KEY
environment variable is set in the build
environment, you can build this with the following command:
$ docker buildx build --secret id=API_KEY .
RUN --mount=type=ssh
This mount type allows the build container to access SSH keys via SSH agents, with support for passphrases.
Option | Description |
---|---|
id | ID of SSH agent socket or key. Defaults to "default". |
target , dst , destination | SSH agent socket path. Defaults to /run/buildkit/ssh_agent.${N} . |
required | If set to true , the instruction errors out when the key is unavailable. Defaults to false . |
mode | File mode for socket in octal. Default 0600 . |
uid | User ID for socket. Default 0 . |
gid | Group ID for socket. Default 0 . |
Example: access to GitLab
# syntax=docker/dockerfile:1
FROM alpine
RUN apk add --no-cache openssh-client
RUN mkdir -p -m 0700 ~/.ssh && ssh-keyscan gitlab.com >> ~/.ssh/known_hosts
RUN --mount=type=ssh \
ssh -q -T git@gitlab.com 2>&1 | tee /hello
# "Welcome to GitLab, @GITLAB_USERNAME_ASSOCIATED_WITH_SSHKEY" should be printed here
# with the type of build progress is defined as `plain`.
$ eval $(ssh-agent)
$ ssh-add ~/.ssh/id_rsa
(Input your passphrase here)
$ docker buildx build --ssh default=$SSH_AUTH_SOCK .
You can also specify a path to *.pem
file on the host directly instead of $SSH_AUTH_SOCK
.
However, pem files with passphrases are not supported.
RUN --network
RUN --network=<TYPE>
RUN --network
allows control over which networking environment the command
is run in.
The supported network types are:
Type | Description |
---|---|
default (default) | Run in the default network. |
none | Run with no network access. |
host | Run in the host's network environment. |
RUN --network=default
Equivalent to not supplying a flag at all, the command is run in the default network for the build.
RUN --network=none
The command is run with no network access (lo
is still available, but is
isolated to this process)
Example: isolating external effects
# syntax=docker/dockerfile:1
FROM python:3.6
ADD mypackage.tgz wheels/
RUN --network=none pip install --find-links wheels mypackage
pip
will only be able to install the packages provided in the tarfile, which
can be controlled by an earlier build stage.
RUN --network=host
The command is run in the host's network environment (similar to
docker build --network=host
, but on a per-instruction basis)
Warning
The use of
--network=host
is protected by thenetwork.host
entitlement, which needs to be enabled when starting the buildkitd daemon with--allow-insecure-entitlement network.host
flag or in buildkitd config, and for a build request with--allow network.host
flag.
RUN --security
Note
Not yet available in stable syntax, use
docker/dockerfile:1-labs
version.
RUN --security=<sandbox|insecure>
The default security mode is sandbox
.
With --security=insecure
, the builder runs the command without sandbox in insecure
mode, which allows to run flows requiring elevated privileges (e.g. containerd).
This is equivalent to running docker run --privileged
.
Warning
In order to access this feature, entitlement
security.insecure
should be enabled when starting the buildkitd daemon with--allow-insecure-entitlement security.insecure
flag or in buildkitd config, and for a build request with--allow security.insecure
flag.
Default sandbox mode can be activated via --security=sandbox
, but that is no-op.
Example: check entitlements
# syntax=docker/dockerfile:1-labs
FROM ubuntu
RUN --security=insecure cat /proc/self/status | grep CapEff
#84 0.093 CapEff: 0000003fffffffff
CMD
The CMD
instruction sets the command to be executed when running a container
from an image.
You can specify CMD
instructions using
shell or exec forms:
CMD ["executable","param1","param2"]
(exec form)CMD ["param1","param2"]
(exec form, as default parameters toENTRYPOINT
)CMD command param1 param2
(shell form)
There can only be one CMD
instruction in a Dockerfile. If you list more than
one CMD
, only the last one takes effect.
The purpose of a CMD
is to provide defaults for an executing container. These
defaults can include an executable, or they can omit the executable, in which
case you must specify an ENTRYPOINT
instruction as well.
If you would like your container to run the same executable every time, then
you should consider using ENTRYPOINT
in combination with CMD
. See
ENTRYPOINT
. If the user specifies arguments to docker run
then they will override the default specified in CMD
, but still use the
default ENTRYPOINT
.
If CMD
is used to provide default arguments for the ENTRYPOINT
instruction,
both the CMD
and ENTRYPOINT
instructions should be specified in the
exec form.
Note
Don't confuse
RUN
withCMD
.RUN
actually runs a command and commits the result;CMD
doesn't execute anything at build time, but specifies the intended command for the image.
LABEL
LABEL <key>=<value> <key>=<value> <key>=<value> ...
The LABEL
instruction adds metadata to an image. A LABEL
is a
key-value pair. To include spaces within a LABEL
value, use quotes and
backslashes as you would in command-line parsing. A few usage examples:
LABEL "com.example.vendor"="ACME Incorporated"
LABEL com.example.label-with-value="foo"
LABEL version="1.0"
LABEL description="This text illustrates \
that label-values can span multiple lines."
An image can have more than one label. You can specify multiple labels on a single line. Prior to Docker 1.10, this decreased the size of the final image, but this is no longer the case. You may still choose to specify multiple labels in a single instruction, in one of the following two ways:
LABEL multi.label1="value1" multi.label2="value2" other="value3"
LABEL multi.label1="value1" \
multi.label2="value2" \
other="value3"
Note
Be sure to use double quotes and not single quotes. Particularly when you are using string interpolation (e.g.
LABEL example="foo-$ENV_VAR"
), single quotes will take the string as is without unpacking the variable's value.
Labels included in base or parent images (images in the FROM
line) are
inherited by your image. If a label already exists but with a different value,
the most-recently-applied value overrides any previously-set value.
To view an image's labels, use the docker image inspect
command. You can use
the --format
option to show just the labels;
$ docker image inspect --format='{{json .Config.Labels}}' myimage
{
"com.example.vendor": "ACME Incorporated",
"com.example.label-with-value": "foo",
"version": "1.0",
"description": "This text illustrates that label-values can span multiple lines.",
"multi.label1": "value1",
"multi.label2": "value2",
"other": "value3"
}
MAINTAINER (deprecated)
MAINTAINER <name>
The MAINTAINER
instruction sets the Author field of the generated images.
The LABEL
instruction is a much more flexible version of this and you should use
it instead, as it enables setting any metadata you require, and can be viewed
easily, for example with docker inspect
. To set a label corresponding to the
MAINTAINER
field you could use:
LABEL org.opencontainers.image.authors="SvenDowideit@home.org.au"
This will then be visible from docker inspect
with the other labels.
EXPOSE
EXPOSE <port> [<port>/<protocol>...]
The EXPOSE
instruction informs Docker that the container listens on the
specified network ports at runtime. You can specify whether the port listens on
TCP or UDP, and the default is TCP if you don't specify a protocol.
The EXPOSE
instruction doesn't actually publish the port. It functions as a
type of documentation between the person who builds the image and the person who
runs the container, about which ports are intended to be published. To
publish the port when running the container, use the -p
flag on docker run
to publish and map one or more ports, or the -P
flag to publish all exposed
ports and map them to high-order ports.
By default, EXPOSE
assumes TCP. You can also specify UDP:
EXPOSE 80/udp
To expose on both TCP and UDP, include two lines:
EXPOSE 80/tcp
EXPOSE 80/udp
In this case, if you use -P
with docker run
, the port will be exposed once
for TCP and once for UDP. Remember that -P
uses an ephemeral high-ordered host
port on the host, so TCP and UDP doesn't use the same port.
Regardless of the EXPOSE
settings, you can override them at runtime by using
the -p
flag. For example
$ docker run -p 80:80/tcp -p 80:80/udp ...
To set up port redirection on the host system, see
using the -P flag.
The docker network
command supports creating networks for communication among
containers without the need to expose or publish specific ports, because the
containers connected to the network can communicate with each other over any
port. For detailed information, see the
overview of this feature.
ENV
ENV <key>=<value> ...
The ENV
instruction sets the environment variable <key>
to the value
<value>
. This value will be in the environment for all subsequent instructions
in the build stage and can be
replaced inline in
many as well. The value will be interpreted for other environment variables, so
quote characters will be removed if they are not escaped. Like command line parsing,
quotes and backslashes can be used to include spaces within values.
Example:
ENV MY_NAME="John Doe"
ENV MY_DOG=Rex\ The\ Dog
ENV MY_CAT=fluffy
The ENV
instruction allows for multiple <key>=<value> ...
variables to be set
at one time, and the example below will yield the same net results in the final
image:
ENV MY_NAME="John Doe" MY_DOG=Rex\ The\ Dog \
MY_CAT=fluffy
The environment variables set using ENV
will persist when a container is run
from the resulting image. You can view the values using docker inspect
, and
change them using docker run --env <key>=<value>
.
A stage inherits any environment variables that were set using ENV
by its
parent stage or any ancestor. Refer to the
multi-stage builds section
in the manual for more information.
Environment variable persistence can cause unexpected side effects. For example,
setting ENV DEBIAN_FRONTEND=noninteractive
changes the behavior of apt-get
,
and may confuse users of your image.
If an environment variable is only needed during build, and not in the final image, consider setting a value for a single command instead:
RUN DEBIAN_FRONTEND=noninteractive apt-get update && apt-get install -y ...
Or using
ARG
, which is not persisted in the final image:
ARG DEBIAN_FRONTEND=noninteractive
RUN apt-get update && apt-get install -y ...
Note
Alternative syntax
The
ENV
instruction also allows an alternative syntaxENV <key> <value>
, omitting the=
. For example:ENV MY_VAR my-value
This syntax does not allow for multiple environment-variables to be set in a single
ENV
instruction, and can be confusing. For example, the following sets a single environment variable (ONE
) with value"TWO= THREE=world"
:ENV ONE TWO= THREE=world
The alternative syntax is supported for backward compatibility, but discouraged for the reasons outlined above, and may be removed in a future release.
ADD
ADD has two forms. The latter form is required for paths containing whitespace.
ADD [OPTIONS] <src> ... <dest>
ADD [OPTIONS] ["<src>", ... "<dest>"]
The available [OPTIONS]
are:
Option | Minimum Dockerfile version |
---|---|
--keep-git-dir | 1.1 |
--checksum | 1.6 |
--chown | |
--chmod | 1.2 |
--link | 1.4 |
--exclude | 1.7 |
The ADD
instruction copies new files or directories from <src>
and adds
them to the filesystem of the image at the path <dest>
. Files and directories
can be copied from the build context, a remote URL, or a Git repository.
The ADD
and COPY
instructions are functionally similar, but serve slightly different purposes.
Learn more about the
differences between ADD
and COPY
.
Source
You can specify multiple source files or directories with ADD
. The last
argument must always be the destination. For example, to add two files,
file1.txt
and file2.txt
, from the build context to /usr/src/things/
in
the build container:
ADD file1.txt file2.txt /usr/src/things/
If you specify multiple source files, either directly or using a wildcard, then
the destination must be a directory (must end with a slash /
).
To add files from a remote location, you can specify a URL or the address of a Git repository as the source. For example:
ADD https://example.com/archive.zip /usr/src/things/
ADD git@github.com:user/repo.git /usr/src/things/
BuildKit detects the type of <src>
and processes it accordingly.
- If
<src>
is a local file or directory, the contents of the directory are copied to the specified destination. See Adding files from the build context. - If
<src>
is a local tar archive, it is decompressed and extracted to the specified destination. See Adding local tar archives. - If
<src>
is a URL, the contents of the URL are downloaded and placed at the specified destination. See Adding files from a URL. - If
<src>
is a Git repository, the repository is cloned to the specified destination. See Adding files from a Git repository.
Adding files from the build context
Any relative or local path that doesn't begin with a http://
, https://
, or
git@
protocol prefix is considered a local file path. The local file path is
relative to the build context. For example, if the build context is the current
directory, ADD file.txt /
adds the file at ./file.txt
to the root of the
filesystem in the build container.
When adding source files from the build context, their paths are interpreted as
relative to the root of the context. If you specify a relative path leading
outside of the build context, such as ADD ../something /something
, parent
directory paths are stripped out automatically. The effective source path in
this example becomes ADD something /something
.
If the source is a directory, the contents of the directory are copied, including filesystem metadata. The directory itself isn't copied, only its contents. If it contains subdirectories, these are also copied, and merged with any existing directories at the destination. Any conflicts are resolved in favor of the content being added, on a file-by-file basis, except if you're trying to copy a directory onto an existing file, in which case an error is raised.
If the source is a file, the file and its metadata are copied to the destination. File permissions are preserved. If the source is a file and a directory with the same name exists at the destination, an error is raised.
If you pass a Dockerfile through stdin to the build (docker build - < Dockerfile
), there is no build context. In this case, you can only use the
ADD
instruction to copy remote files. You can also pass a tar archive through
stdin: (docker build - < archive.tar
), the Dockerfile at the root of the
archive and the rest of the archive will be used as the context of the build.
Pattern matching
For local files, each <src>
may contain wildcards and matching will be done
using Go's
filepath.Match rules.
For example, to add all files and directories in the root of the build context
ending with .png
:
ADD *.png /dest/
In the following example, ?
is a single-character wildcard, matching e.g.
index.js
and index.ts
.
ADD index.?s /dest/
When adding files or directories that contain special characters (such as [
and ]
), you need to escape those paths following the Golang rules to prevent
them from being treated as a matching pattern. For example, to add a file
named arr[0].txt
, use the following;
ADD arr[[]0].txt /dest/
Adding local tar archives
When using a local tar archive as the source for ADD
, and the archive is in a
recognized compression format (gzip
, bzip2
or xz
, or uncompressed), the
archive is decompressed and extracted into the specified destination. Only
local tar archives are extracted. If the tar archive is a remote URL, the
archive is not extracted, but downloaded and placed at the destination.
When a directory is extracted, it has the same behavior as tar -x
.
The result is the union of:
- Whatever existed at the destination path, and
- The contents of the source tree, with conflicts resolved in favor of the content being added, on a file-by-file basis.
Note
Whether a file is identified as a recognized compression format or not is done solely based on the contents of the file, not the name of the file. For example, if an empty file happens to end with
.tar.gz
this isn't recognized as a compressed file and doesn't generate any kind of decompression error message, rather the file will simply be copied to the destination.
Adding files from a URL
In the case where source is a remote file URL, the destination will have
permissions of 600. If the HTTP response contains a Last-Modified
header, the
timestamp from that header will be used to set the mtime
on the destination
file. However, like any other file processed during an ADD
, mtime
isn't
included in the determination of whether or not the file has changed and the
cache should be updated.
If the destination ends with a trailing slash, then the filename is inferred
from the URL path. For example, ADD http://example.com/foobar /
would create
the file /foobar
. The URL must have a nontrivial path so that an appropriate
filename can be discovered (http://example.com
doesn't work).
If the destination doesn't end with a trailing slash, the destination path
becomes the filename of the file downloaded from the URL. For example, ADD http://example.com/foo /bar
creates the file /bar
.
If your URL files are protected using authentication, you need to use RUN wget
,
RUN curl
or use another tool from within the container as the ADD
instruction
doesn't support authentication.
Adding files from a Git repository
To use a Git repository as the source for ADD
, you can reference the
repository's HTTP or SSH address as the source. The repository is cloned to the
specified destination in the image.
ADD https://github.com/user/repo.git /mydir/
You can use URL fragments to specify a specific branch, tag, commit, or
subdirectory. For example, to add the docs
directory of the v0.14.1
tag of
the buildkit
repository:
ADD git@github.com:moby/buildkit.git#v0.14.1:docs /buildkit-docs
For more information about Git URL fragments, see URL fragments.
When adding from a Git repository, the permissions bits for files are 644. If a file in the repository has the executable bit set, it will have permissions set to 755. Directories have permissions set to 755.
When using a Git repository as the source, the repository must be accessible from the build context. To add a repository via SSH, whether public or private, you must pass an SSH key for authentication. For example, given the following Dockerfile:
# syntax=docker/dockerfile:1
FROM alpine
ADD git@git.example.com:foo/bar.git /bar
To build this Dockerfile, pass the --ssh
flag to the docker build
to mount
the SSH agent socket to the build. For example:
$ docker build --ssh default .
For more information about building with secrets, see Build secrets.
Destination
If the destination path begins with a forward slash, it's interpreted as an absolute path, and the source files are copied into the specified destination relative to the root of the current build stage.
# create /abs/test.txt
ADD test.txt /abs/
Trailing slashes are significant. For example, ADD test.txt /abs
creates a
file at /abs
, whereas ADD test.txt /abs/
creates /abs/test.txt
.
If the destination path doesn't begin with a leading slash, it's interpreted as relative to the working directory of the build container.
WORKDIR /usr/src/app
# create /usr/src/app/rel/test.txt
ADD test.txt rel/
If destination doesn't exist, it's created, along with all missing directories in its path.
If the source is a file, and the destination doesn't end with a trailing slash, the source file will be written to the destination path as a file.
ADD --keep-git-dir
ADD [--keep-git-dir=<boolean>] <src> ... <dir>
When <src>
is the HTTP or SSH address of a remote Git repository,
BuildKit adds the contents of the Git repository to the image
excluding the .git
directory by default.
The --keep-git-dir=true
flag lets you preserve the .git
directory.
# syntax=docker/dockerfile:1
FROM alpine
ADD --keep-git-dir=true https://github.com/moby/buildkit.git#v0.10.1 /buildkit
ADD --checksum
ADD [--checksum=<hash>] <src> ... <dir>
The --checksum
flag lets you verify the checksum of a remote resource. The
checksum is formatted as <algorithm>:<hash>
. The supported algorithms are
sha256
, sha384
, and sha512
.
ADD --checksum=sha256:24454f830cdb571e2c4ad15481119c43b3cafd48dd869a9b2945d1036d1dc68d https://mirrors.edge.kernel.org/pub/linux/kernel/Historic/linux-0.01.tar.gz /
The --checksum
flag only supports HTTP(S) sources.
ADD --chown --chmod
See
COPY --chown --chmod
.
ADD --link
See
COPY --link
.
ADD --exclude
See
COPY --exclude
.
COPY
COPY has two forms. The latter form is required for paths containing whitespace.
COPY [OPTIONS] <src> ... <dest>
COPY [OPTIONS] ["<src>", ... "<dest>"]
The available [OPTIONS]
are:
Option | Minimum Dockerfile version |
---|---|
--from | |
--chown | |
--chmod | 1.2 |
--link | 1.4 |
--parents | 1.7 |
--exclude | 1.7 |
The COPY
instruction copies new files or directories from <src>
and adds
them to the filesystem of the image at the path <dest>
. Files and directories
can be copied from the build context, build stage, named context, or an image.
The ADD
and COPY
instructions are functionally similar, but serve slightly different purposes.
Learn more about the
differences between ADD
and COPY
.
Source
You can specify multiple source files or directories with COPY
. The last
argument must always be the destination. For example, to copy two files,
file1.txt
and file2.txt
, from the build context to /usr/src/things/
in
the build container:
COPY file1.txt file2.txt /usr/src/things/
If you specify multiple source files, either directly or using a wildcard, then
the destination must be a directory (must end with a slash /
).
COPY
accepts a flag --from=<name>
that lets you specify the source location
to be a build stage, context, or image. The following example copies files from
a stage named build
:
FROM golang AS build
WORKDIR /app
RUN --mount=type=bind,target=. go build -o /myapp ./cmd
COPY --from=build /myapp /usr/bin/
For more information about copying from named sources, see the
--from
flag.
Copying from the build context
When copying source files from the build context, their paths are interpreted as
relative to the root of the context. If you specify a relative path leading
outside of the build context, such as COPY ../something /something
, parent
directory paths are stripped out automatically. The effective source path in
this example becomes COPY something /something
.
If the source is a directory, the contents of the directory are copied, including filesystem metadata. The directory itself isn't copied, only its contents. If it contains subdirectories, these are also copied, and merged with any existing directories at the destination. Any conflicts are resolved in favor of the content being added, on a file-by-file basis, except if you're trying to copy a directory onto an existing file, in which case an error is raised.
If the source is a file, the file and its metadata are copied to the destination. File permissions are preserved. If the source is a file and a directory with the same name exists at the destination, an error is raised.
If you pass a Dockerfile through stdin to the build (docker build - < Dockerfile
), there is no build context. In this case, you can only use the
COPY
instruction to copy files from other stages, named contexts, or images,
using the
--from
flag. You can also pass a tar archive
through stdin: (docker build - < archive.tar
), the Dockerfile at the root of
the archive and the rest of the archive will be used as the context of the
build.
When using a Git repository as the build context, the permissions bits for copied files are 644. If a file in the repository has the executable bit set, it will have permissions set to 755. Directories have permissions set to 755.
Pattern matching
For local files, each <src>
may contain wildcards and matching will be done
using Go's
filepath.Match rules.
For example, to add all files and directories in the root of the build context
ending with .png
:
COPY *.png /dest/
In the following example, ?
is a single-character wildcard, matching e.g.
index.js
and index.ts
.
COPY index.?s /dest/
When adding files or directories that contain special characters (such as [
and ]
), you need to escape those paths following the Golang rules to prevent
them from being treated as a matching pattern. For example, to add a file
named arr[0].txt
, use the following;
COPY arr[[]0].txt /dest/
Destination
If the destination path begins with a forward slash, it's interpreted as an absolute path, and the source files are copied into the specified destination relative to the root of the current build stage.
# create /abs/test.txt
COPY test.txt /abs/
Trailing slashes are significant. For example, COPY test.txt /abs
creates a
file at /abs
, whereas COPY test.txt /abs/
creates /abs/test.txt
.
If the destination path doesn't begin with a leading slash, it's interpreted as relative to the working directory of the build container.
WORKDIR /usr/src/app
# create /usr/src/app/rel/test.txt
COPY test.txt rel/
If destination doesn't exist, it's created, along with all missing directories in its path.
If the source is a file, and the destination doesn't end with a trailing slash, the source file will be written to the destination path as a file.
COPY --from
By default, the COPY
instruction copies files from the build context. The
COPY --from
flag lets you copy files from an image, a build stage,
or a named context instead.
COPY [--from=<image|stage|context>] <src> ... <dest>
To copy from a build stage in a
multi-stage build,
specify the name of the stage you want to copy from. You specify stage names
using the AS
keyword with the FROM
instruction.
# syntax=docker/dockerfile:1
FROM alpine AS build
COPY . .
RUN apk add clang
RUN clang -o /hello hello.c
FROM scratch
COPY --from=build /hello /
You can also copy files directly from named contexts (specified with
--build-context <name>=<source>
) or images. The following example copies an
nginx.conf
file from the official Nginx image.
COPY --from=nginx:latest /etc/nginx/nginx.conf /nginx.conf
The source path of COPY --from
is always resolved from filesystem root of the
image or stage that you specify.
COPY --chown --chmod
Note
Only octal notation is currently supported. Non-octal support is tracked in moby/buildkit#1951.
COPY [--chown=<user>:<group>] [--chmod=<perms> ...] <src> ... <dest>
The --chown
and --chmod
features are only supported on Dockerfiles used to build Linux containers,
and doesn't work on Windows containers. Since user and group ownership concepts do
not translate between Linux and Windows, the use of /etc/passwd
and /etc/group
for
translating user and group names to IDs restricts this feature to only be viable for
Linux OS-based containers.
All files and directories copied from the build context are created with a UID and GID of 0
unless the
optional --chown
flag specifies a given username, groupname, or UID/GID
combination to request specific ownership of the copied content. The
format of the --chown
flag allows for either username and groupname strings
or direct integer UID and GID in any combination. Providing a username without
groupname or a UID without GID will use the same numeric UID as the GID. If a
username or groupname is provided, the container's root filesystem
/etc/passwd
and /etc/group
files will be used to perform the translation
from name to integer UID or GID respectively. The following examples show
valid definitions for the --chown
flag:
COPY --chown=55:mygroup files* /somedir/
COPY --chown=bin files* /somedir/
COPY --chown=1 files* /somedir/
COPY --chown=10:11 files* /somedir/
COPY --chown=myuser:mygroup --chmod=644 files* /somedir/
If the container root filesystem doesn't contain either /etc/passwd
or
/etc/group
files and either user or group names are used in the --chown
flag, the build will fail on the COPY
operation. Using numeric IDs requires
no lookup and does not depend on container root filesystem content.
With the Dockerfile syntax version 1.10.0 and later,
the --chmod
flag supports variable interpolation,
which lets you define the permission bits using build arguments:
# syntax=docker/dockerfile:1.10
FROM alpine
WORKDIR /src
ARG MODE=440
COPY --chmod=$MODE . .
COPY --link
COPY [--link[=<boolean>]] <src> ... <dest>
Enabling this flag in COPY
or ADD
commands allows you to copy files with
enhanced semantics where your files remain independent on their own layer and
don't get invalidated when commands on previous layers are changed.
When --link
is used your source files are copied into an empty destination
directory. That directory is turned into a layer that is linked on top of your
previous state.
# syntax=docker/dockerfile:1
FROM alpine
COPY --link /foo /bar
Is equivalent of doing two builds:
FROM alpine
and
FROM scratch
COPY /foo /bar
and merging all the layers of both images together.
Benefits of using --link
Use --link
to reuse already built layers in subsequent builds with
--cache-from
even if the previous layers have changed. This is especially
important for multi-stage builds where a COPY --from
statement would
previously get invalidated if any previous commands in the same stage changed,
causing the need to rebuild the intermediate stages again. With --link
the
layer the previous build generated is reused and merged on top of the new
layers. This also means you can easily rebase your images when the base images
receive updates, without having to execute the whole build again. In backends
that support it, BuildKit can do this rebase action without the need to push or
pull any layers between the client and the registry. BuildKit will detect this
case and only create new image manifest that contains the new layers and old
layers in correct order.
The same behavior where BuildKit can avoid pulling down the base image can also
happen when using --link
and no other commands that would require access to
the files in the base image. In that case BuildKit will only build the layers
for the COPY
commands and push them to the registry directly on top of the
layers of the base image.
Incompatibilities with --link=false
When using --link
the COPY/ADD
commands are not allowed to read any files
from the previous state. This means that if in previous state the destination
directory was a path that contained a symlink, COPY/ADD
can not follow it.
In the final image the destination path created with --link
will always be a
path containing only directories.
If you don't rely on the behavior of following symlinks in the destination
path, using --link
is always recommended. The performance of --link
is
equivalent or better than the default behavior and, it creates much better
conditions for cache reuse.
COPY --parents
Note
Not yet available in stable syntax, use
docker/dockerfile:1.7-labs
version.
COPY [--parents[=<boolean>]] <src> ... <dest>
The --parents
flag preserves parent directories for src
entries. This flag defaults to false
.
# syntax=docker/dockerfile:1.7-labs
FROM scratch
COPY ./x/a.txt ./y/a.txt /no_parents/
COPY --parents ./x/a.txt ./y/a.txt /parents/
# /no_parents/a.txt
# /parents/x/a.txt
# /parents/y/a.txt
This behavior is similar to the
Linux cp
utility's
--parents
or
rsync
--relative
flag.
As with Rsync, it is possible to limit which parent directories are preserved by
inserting a dot and a slash (./
) into the source path. If such point exists, only parent
directories after it will be preserved. This may be especially useful copies between stages
with --from
where the source paths need to be absolute.
# syntax=docker/dockerfile:1.7-labs
FROM scratch
COPY --parents ./x/./y/*.txt /parents/
# Build context:
# ./x/y/a.txt
# ./x/y/b.txt
#
# Output:
# /parents/y/a.txt
# /parents/y/b.txt
Note that, without the --parents
flag specified, any filename collision will
fail the Linux cp
operation with an explicit error message
(cp: will not overwrite just-created './x/a.txt' with './y/a.txt'
), where the
Buildkit will silently overwrite the target file at the destination.
While it is possible to preserve the directory structure for COPY
instructions consisting of only one src
entry, usually it is more beneficial
to keep the layer count in the resulting image as low as possible. Therefore,
with the --parents
flag, the Buildkit is capable of packing multiple
COPY
instructions together, keeping the directory structure intact.
COPY --exclude
Note
Not yet available in stable syntax, use
docker/dockerfile:1.7-labs
version.
COPY [--exclude=<path> ...] <src> ... <dest>
The --exclude
flag lets you specify a path expression for files to be excluded.
The path expression follows the same format as <src>
,
supporting wildcards and matching using Go's
filepath.Match rules.
For example, to add all files starting with "hom", excluding files with a .txt
extension:
COPY --exclude=*.txt hom* /mydir/
You can specify the --exclude
option multiple times for a COPY
instruction.
Multiple --excludes
are files matching its patterns not to be copied,
even if the files paths match the pattern specified in <src>
.
To add all files starting with "hom", excluding files with either .txt
or .md
extensions:
COPY --exclude=*.txt --exclude=*.md hom* /mydir/
ENTRYPOINT
An ENTRYPOINT
allows you to configure a container that will run as an executable.
ENTRYPOINT
has two possible forms:
The exec form, which is the preferred form:
ENTRYPOINT ["executable", "param1", "param2"]
The shell form:
ENTRYPOINT command param1 param2
For more information about the different forms, see Shell and exec form.
The following command starts a container from the nginx
with its default
content, listening on port 80:
$ docker run -i -t --rm -p 80:80 nginx
Command line arguments to docker run <image>
will be appended after all
elements in an exec form ENTRYPOINT
, and will override all elements specified
using CMD
.
This allows arguments to be passed to the entry point, i.e., docker run <image> -d
will pass the -d
argument to the entry point. You can override
the ENTRYPOINT
instruction using the docker run --entrypoint
flag.
The shell form of ENTRYPOINT
prevents any CMD
command line arguments from
being used. It also starts your ENTRYPOINT
as a subcommand of /bin/sh -c
,
which does not pass signals. This means that the executable will not be the
container's PID 1
, and will not receive Unix signals. In this case, your
executable doesn't receive a SIGTERM
from docker stop <container>
.
Only the last ENTRYPOINT
instruction in the Dockerfile will have an effect.
Exec form ENTRYPOINT example
You can use the exec form of ENTRYPOINT
to set fairly stable default commands
and arguments and then use either form of CMD
to set additional defaults that
are more likely to be changed.
FROM ubuntu
ENTRYPOINT ["top", "-b"]
CMD ["-c"]
When you run the container, you can see that top
is the only process:
$ docker run -it --rm --name test top -H
top - 08:25:00 up 7:27, 0 users, load average: 0.00, 0.01, 0.05
Threads: 1 total, 1 running, 0 sleeping, 0 stopped, 0 zombie
%Cpu(s): 0.1 us, 0.1 sy, 0.0 ni, 99.7 id, 0.0 wa, 0.0 hi, 0.0 si, 0.0 st
KiB Mem: 2056668 total, 1616832 used, 439836 free, 99352 buffers
KiB Swap: 1441840 total, 0 used, 1441840 free. 1324440 cached Mem
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
1 root 20 0 19744 2336 2080 R 0.0 0.1 0:00.04 top
To examine the result further, you can use docker exec
:
$ docker exec -it test ps aux
USER PID %CPU %MEM VSZ RSS TTY STAT START TIME COMMAND
root 1 2.6 0.1 19752 2352 ? Ss+ 08:24 0:00 top -b -H
root 7 0.0 0.1 15572 2164 ? R+ 08:25 0:00 ps aux
And you can gracefully request top
to shut down using docker stop test
.
The following Dockerfile shows using the ENTRYPOINT
to run Apache in the
foreground (i.e., as PID 1
):
FROM debian:stable
RUN apt-get update && apt-get install -y --force-yes apache2
EXPOSE 80 443
VOLUME ["/var/www", "/var/log/apache2", "/etc/apache2"]
ENTRYPOINT ["/usr/sbin/apache2ctl", "-D", "FOREGROUND"]
If you need to write a starter script for a single executable, you can ensure that
the final executable receives the Unix signals by using exec
and gosu
commands:
#!/usr/bin/env bash
set -e
if [ "$1" = 'postgres' ]; then
chown -R postgres "$PGDATA"
if [ -z "$(ls -A "$PGDATA")" ]; then
gosu postgres initdb
fi
exec gosu postgres "$@"
fi
exec "$@"
Lastly, if you need to do some extra cleanup (or communicate with other containers)
on shutdown, or are co-ordinating more than one executable, you may need to ensure
that the ENTRYPOINT
script receives the Unix signals, passes them on, and then
does some more work:
#!/bin/sh
# Note: I've written this using sh so it works in the busybox container too
# USE the trap if you need to also do manual cleanup after the service is stopped,
# or need to start multiple services in the one container
trap "echo TRAPed signal" HUP INT QUIT TERM
# start service in background here
/usr/sbin/apachectl start
echo "[hit enter key to exit] or run 'docker stop <container>'"
read
# stop service and clean up here
echo "stopping apache"
/usr/sbin/apachectl stop
echo "exited $0"
If you run this image with docker run -it --rm -p 80:80 --name test apache
,
you can then examine the container's processes with docker exec
, or docker top
,
and then ask the script to stop Apache:
$ docker exec -it test ps aux
USER PID %CPU %MEM VSZ RSS TTY STAT START TIME COMMAND
root 1 0.1 0.0 4448 692 ? Ss+ 00:42 0:00 /bin/sh /run.sh 123 cmd cmd2
root 19 0.0 0.2 71304 4440 ? Ss 00:42 0:00 /usr/sbin/apache2 -k start
www-data 20 0.2 0.2 360468 6004 ? Sl 00:42 0:00 /usr/sbin/apache2 -k start
www-data 21 0.2 0.2 360468 6000 ? Sl 00:42 0:00 /usr/sbin/apache2 -k start
root 81 0.0 0.1 15572 2140 ? R+ 00:44 0:00 ps aux
$ docker top test
PID USER COMMAND
10035 root {run.sh} /bin/sh /run.sh 123 cmd cmd2
10054 root /usr/sbin/apache2 -k start
10055 33 /usr/sbin/apache2 -k start
10056 33 /usr/sbin/apache2 -k start
$ /usr/bin/time docker stop test
test
real 0m 0.27s
user 0m 0.03s
sys 0m 0.03s
Note
You can override the
ENTRYPOINT
setting using--entrypoint
, but this can only set the binary to exec (nosh -c
will be used).
Shell form ENTRYPOINT example
You can specify a plain string for the ENTRYPOINT
and it will execute in /bin/sh -c
.
This form will use shell processing to substitute shell environment variables,
and will ignore any CMD
or docker run
command line arguments.
To ensure that docker stop
will signal any long running ENTRYPOINT
executable
correctly, you need to remember to start it with exec
:
FROM ubuntu
ENTRYPOINT exec top -b
When you run this image, you'll see the single PID 1
process:
$ docker run -it --rm --name test top
Mem: 1704520K used, 352148K free, 0K shrd, 0K buff, 140368121167873K cached
CPU: 5% usr 0% sys 0% nic 94% idle 0% io 0% irq 0% sirq
Load average: 0.08 0.03 0.05 2/98 6
PID PPID USER STAT VSZ %VSZ %CPU COMMAND
1 0 root R 3164 0% 0% top -b
Which exits cleanly on docker stop
:
$ /usr/bin/time docker stop test
test
real 0m 0.20s
user 0m 0.02s
sys 0m 0.04s
If you forget to add exec
to the beginning of your ENTRYPOINT
:
FROM ubuntu
ENTRYPOINT top -b
CMD -- --ignored-param1
You can then run it (giving it a name for the next step):
$ docker run -it --name test top --ignored-param2
top - 13:58:24 up 17 min, 0 users, load average: 0.00, 0.00, 0.00
Tasks: 2 total, 1 running, 1 sleeping, 0 stopped, 0 zombie
%Cpu(s): 16.7 us, 33.3 sy, 0.0 ni, 50.0 id, 0.0 wa, 0.0 hi, 0.0 si, 0.0 st
MiB Mem : 1990.8 total, 1354.6 free, 231.4 used, 404.7 buff/cache
MiB Swap: 1024.0 total, 1024.0 free, 0.0 used. 1639.8 avail Mem
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
1 root 20 0 2612 604 536 S 0.0 0.0 0:00.02 sh
6 root 20 0 5956 3188 2768 R 0.0 0.2 0:00.00 top
You can see from the output of top
that the specified ENTRYPOINT
is not PID 1
.
If you then run docker stop test
, the container will not exit cleanly - the
stop
command will be forced to send a SIGKILL
after the timeout:
$ docker exec -it test ps waux
USER PID %CPU %MEM VSZ RSS TTY STAT START TIME COMMAND
root 1 0.4 0.0 2612 604 pts/0 Ss+ 13:58 0:00 /bin/sh -c top -b --ignored-param2
root 6 0.0 0.1 5956 3188 pts/0 S+ 13:58 0:00 top -b
root 7 0.0 0.1 5884 2816 pts/1 Rs+ 13:58 0:00 ps waux
$ /usr/bin/time docker stop test
test
real 0m 10.19s
user 0m 0.04s
sys 0m 0.03s
Understand how CMD and ENTRYPOINT interact
Both CMD
and ENTRYPOINT
instructions define what command gets executed when running a container.
There are few rules that describe their co-operation.
Dockerfile should specify at least one of
CMD
orENTRYPOINT
commands.ENTRYPOINT
should be defined when using the container as an executable.CMD
should be used as a way of defining default arguments for anENTRYPOINT
command or for executing an ad-hoc command in a container.CMD
will be overridden when running the container with alternative arguments.
The table below shows what command is executed for different ENTRYPOINT
/ CMD
combinations:
No ENTRYPOINT | ENTRYPOINT exec_entry p1_entry | ENTRYPOINT ["exec_entry", "p1_entry"] | |
---|---|---|---|
No CMD | error, not allowed | /bin/sh -c exec_entry p1_entry | exec_entry p1_entry |
CMD ["exec_cmd", "p1_cmd"] | exec_cmd p1_cmd | /bin/sh -c exec_entry p1_entry | exec_entry p1_entry exec_cmd p1_cmd |
CMD exec_cmd p1_cmd | /bin/sh -c exec_cmd p1_cmd | /bin/sh -c exec_entry p1_entry | exec_entry p1_entry /bin/sh -c exec_cmd p1_cmd |
Note
If
CMD
is defined from the base image, settingENTRYPOINT
will resetCMD
to an empty value. In this scenario,CMD
must be defined in the current image to have a value.
VOLUME
VOLUME ["/data"]
The VOLUME
instruction creates a mount point with the specified name
and marks it as holding externally mounted volumes from native host or other
containers. The value can be a JSON array, VOLUME ["/var/log/"]
, or a plain
string with multiple arguments, such as VOLUME /var/log
or VOLUME /var/log /var/db
. For more information/examples and mounting instructions via the
Docker client, refer to
Share Directories via Volumes
documentation.
The docker run
command initializes the newly created volume with any data
that exists at the specified location within the base image. For example,
consider the following Dockerfile snippet:
FROM ubuntu
RUN mkdir /myvol
RUN echo "hello world" > /myvol/greeting
VOLUME /myvol
This Dockerfile results in an image that causes docker run
to
create a new mount point at /myvol
and copy the greeting
file
into the newly created volume.
Notes about specifying volumes
Keep the following things in mind about volumes in the Dockerfile.
Volumes on Windows-based containers: When using Windows-based containers, the destination of a volume inside the container must be one of:
- a non-existing or empty directory
- a drive other than
C:
Changing the volume from within the Dockerfile: If any build steps change the data within the volume after it has been declared, those changes will be discarded.
JSON formatting: The list is parsed as a JSON array. You must enclose words with double quotes (
"
) rather than single quotes ('
).The host directory is declared at container run-time: The host directory (the mountpoint) is, by its nature, host-dependent. This is to preserve image portability, since a given host directory can't be guaranteed to be available on all hosts. For this reason, you can't mount a host directory from within the Dockerfile. The
VOLUME
instruction does not support specifying ahost-dir
parameter. You must specify the mountpoint when you create or run the container.
USER
USER <user>[:<group>]
or
USER <UID>[:<GID>]
The USER
instruction sets the user name (or UID) and optionally the user
group (or GID) to use as the default user and group for the remainder of the
current stage. The specified user is used for RUN
instructions and at
runtime, runs the relevant ENTRYPOINT
and CMD
commands.
Note that when specifying a group for the user, the user will have only the specified group membership. Any other configured group memberships will be ignored.
Warning
When the user doesn't have a primary group then the image (or the next instructions) will be run with the
root
group.On Windows, the user must be created first if it's not a built-in account. This can be done with the
net user
command called as part of a Dockerfile.
FROM microsoft/windowsservercore
# Create Windows user in the container
RUN net user /add patrick
# Set it for subsequent commands
USER patrick
WORKDIR
WORKDIR /path/to/workdir
The WORKDIR
instruction sets the working directory for any RUN
, CMD
,
ENTRYPOINT
, COPY
and ADD
instructions that follow it in the Dockerfile.
If the WORKDIR
doesn't exist, it will be created even if it's not used in any
subsequent Dockerfile instruction.
The WORKDIR
instruction can be used multiple times in a Dockerfile. If a
relative path is provided, it will be relative to the path of the previous
WORKDIR
instruction. For example:
WORKDIR /a
WORKDIR b
WORKDIR c
RUN pwd
The output of the final pwd
command in this Dockerfile would be /a/b/c
.
The WORKDIR
instruction can resolve environment variables previously set using
ENV
. You can only use environment variables explicitly set in the Dockerfile.
For example:
ENV DIRPATH=/path
WORKDIR $DIRPATH/$DIRNAME
RUN pwd
The output of the final pwd
command in this Dockerfile would be
/path/$DIRNAME
If not specified, the default working directory is /
. In practice, if you aren't building a Dockerfile from scratch (FROM scratch
),
the WORKDIR
may likely be set by the base image you're using.
Therefore, to avoid unintended operations in unknown directories, it's best practice to set your WORKDIR
explicitly.
ARG
ARG <name>[=<default value>]
The ARG
instruction defines a variable that users can pass at build-time to
the builder with the docker build
command using the --build-arg <varname>=<value>
flag.
Warning
It isn't recommended to use build arguments for passing secrets such as user credentials, API tokens, etc. Build arguments are visible in the
docker history
command and inmax
mode provenance attestations, which are attached to the image by default if you use the Buildx GitHub Actions and your GitHub repository is public.Refer to the
RUN --mount=type=secret
section to learn about secure ways to use secrets when building images.
A Dockerfile may include one or more ARG
instructions. For example,
the following is a valid Dockerfile:
FROM busybox
ARG user1
ARG buildno
# ...
Default values
An ARG
instruction can optionally include a default value:
FROM busybox
ARG user1=someuser
ARG buildno=1
# ...
If an ARG
instruction has a default value and if there is no value passed
at build-time, the builder uses the default.
Scope
An ARG
variable definition comes into effect from the line on which it is
defined in the Dockerfile not from the argument's use on the command-line or
elsewhere. For example, consider this Dockerfile:
FROM busybox
USER ${username:-some_user}
ARG username
USER $username
# ...
A user builds this file by calling:
$ docker build --build-arg username=what_user .
The USER
at line 2 evaluates to some_user
as the username
variable is defined on the
subsequent line 3. The USER
at line 4 evaluates to what_user
, as the username
argument is
defined and the what_user
value was passed on the command line. Prior to its definition by an
ARG
instruction, any use of a variable results in an empty string.
An ARG
instruction goes out of scope at the end of the build
stage where it was defined. To use an argument in multiple stages, each stage must
include the ARG
instruction.
FROM busybox
ARG SETTINGS
RUN ./run/setup $SETTINGS
FROM busybox
ARG SETTINGS
RUN ./run/other $SETTINGS
Using ARG variables
You can use an ARG
or an ENV
instruction to specify variables that are
available to the RUN
instruction. Environment variables defined using the
ENV
instruction always override an ARG
instruction of the same name. Consider
this Dockerfile with an ENV
and ARG
instruction.
FROM ubuntu
ARG CONT_IMG_VER
ENV CONT_IMG_VER=v1.0.0
RUN echo $CONT_IMG_VER
Then, assume this image is built with this command:
$ docker build --build-arg CONT_IMG_VER=v2.0.1 .
In this case, the RUN
instruction uses v1.0.0
instead of the ARG
setting
passed by the user:v2.0.1
This behavior is similar to a shell
script where a locally scoped variable overrides the variables passed as
arguments or inherited from environment, from its point of definition.
Using the example above but a different ENV
specification you can create more
useful interactions between ARG
and ENV
instructions:
FROM ubuntu
ARG CONT_IMG_VER
ENV CONT_IMG_VER=${CONT_IMG_VER:-v1.0.0}
RUN echo $CONT_IMG_VER
Unlike an ARG
instruction, ENV
values are always persisted in the built
image. Consider a docker build without the --build-arg
flag:
$ docker build .
Using this Dockerfile example, CONT_IMG_VER
is still persisted in the image but
its value would be v1.0.0
as it is the default set in line 3 by the ENV
instruction.
The variable expansion technique in this example allows you to pass arguments
from the command line and persist them in the final image by leveraging the
ENV
instruction. Variable expansion is only supported for
a limited set of
Dockerfile instructions.
Predefined ARGs
Docker has a set of predefined ARG
variables that you can use without a
corresponding ARG
instruction in the Dockerfile.
HTTP_PROXY
http_proxy
HTTPS_PROXY
https_proxy
FTP_PROXY
ftp_proxy
NO_PROXY
no_proxy
ALL_PROXY
all_proxy
To use these, pass them on the command line using the --build-arg
flag, for
example:
$ docker build --build-arg HTTPS_PROXY=https://my-proxy.example.com .
By default, these pre-defined variables are excluded from the output of
docker history
. Excluding them reduces the risk of accidentally leaking
sensitive authentication information in an HTTP_PROXY
variable.
For example, consider building the following Dockerfile using
--build-arg HTTP_PROXY=http://user:pass@proxy.lon.example.com
FROM ubuntu
RUN echo "Hello World"
In this case, the value of the HTTP_PROXY
variable is not available in the
docker history
and is not cached. If you were to change location, and your
proxy server changed to http://user:pass@proxy.sfo.example.com
, a subsequent
build does not result in a cache miss.
If you need to override this behaviour then you may do so by adding an ARG
statement in the Dockerfile as follows:
FROM ubuntu
ARG HTTP_PROXY
RUN echo "Hello World"
When building this Dockerfile, the HTTP_PROXY
is preserved in the
docker history
, and changing its value invalidates the build cache.
Automatic platform ARGs in the global scope
This feature is only available when using the BuildKit backend.
BuildKit supports a predefined set of ARG
variables with information on the platform of
the node performing the build (build platform) and on the platform of the
resulting image (target platform). The target platform can be specified with
the --platform
flag on docker build
.
The following ARG
variables are set automatically:
TARGETPLATFORM
- platform of the build result. Eglinux/amd64
,linux/arm/v7
,windows/amd64
.TARGETOS
- OS component of TARGETPLATFORMTARGETARCH
- architecture component of TARGETPLATFORMTARGETVARIANT
- variant component of TARGETPLATFORMBUILDPLATFORM
- platform of the node performing the build.BUILDOS
- OS component of BUILDPLATFORMBUILDARCH
- architecture component of BUILDPLATFORMBUILDVARIANT
- variant component of BUILDPLATFORM
These arguments are defined in the global scope so are not automatically
available inside build stages or for your RUN
commands. To expose one of
these arguments inside the build stage redefine it without value.
For example:
FROM alpine
ARG TARGETPLATFORM
RUN echo "I'm building for $TARGETPLATFORM"
BuildKit built-in build args
Arg | Type | Description |
---|---|---|
BUILDKIT_CACHE_MOUNT_NS | String | Set optional cache ID namespace. |
BUILDKIT_CONTEXT_KEEP_GIT_DIR | Bool | Trigger Git context to keep the .git directory. |
BUILDKIT_INLINE_CACHE 2 | Bool | Inline cache metadata to image config or not. |
BUILDKIT_MULTI_PLATFORM | Bool | Opt into deterministic output regardless of multi-platform output or not. |
BUILDKIT_SANDBOX_HOSTNAME | String | Set the hostname (default buildkitsandbox ) |
BUILDKIT_SYNTAX | String | Set frontend image |
SOURCE_DATE_EPOCH | Int | Set the Unix timestamp for created image and layers. More info from reproducible builds. Supported since Dockerfile 1.5, BuildKit 0.11 |
Example: keep .git
dir
When using a Git context, .git
dir is not kept on checkouts. It can be
useful to keep it around if you want to retrieve git information during
your build:
# syntax=docker/dockerfile:1
FROM alpine
WORKDIR /src
RUN --mount=target=. \
make REVISION=$(git rev-parse HEAD) build
$ docker build --build-arg BUILDKIT_CONTEXT_KEEP_GIT_DIR=1 https://github.com/user/repo.git#main
Impact on build caching
ARG
variables are not persisted into the built image as ENV
variables are.
However, ARG
variables do impact the build cache in similar ways. If a
Dockerfile defines an ARG
variable whose value is different from a previous
build, then a "cache miss" occurs upon its first usage, not its definition. In
particular, all RUN
instructions following an ARG
instruction use the ARG
variable implicitly (as an environment variable), thus can cause a cache miss.
All predefined ARG
variables are exempt from caching unless there is a
matching ARG
statement in the Dockerfile.
For example, consider these two Dockerfile:
FROM ubuntu
ARG CONT_IMG_VER
RUN echo $CONT_IMG_VER
FROM ubuntu
ARG CONT_IMG_VER
RUN echo hello
If you specify --build-arg CONT_IMG_VER=<value>
on the command line, in both
cases, the specification on line 2 doesn't cause a cache miss; line 3 does
cause a cache miss. ARG CONT_IMG_VER
causes the RUN
line to be identified
as the same as running CONT_IMG_VER=<value> echo hello
, so if the <value>
changes, you get a cache miss.
Consider another example under the same command line:
FROM ubuntu
ARG CONT_IMG_VER
ENV CONT_IMG_VER=$CONT_IMG_VER
RUN echo $CONT_IMG_VER
In this example, the cache miss occurs on line 3. The miss happens because
the variable's value in the ENV
references the ARG
variable and that
variable is changed through the command line. In this example, the ENV
command causes the image to include the value.
If an ENV
instruction overrides an ARG
instruction of the same name, like
this Dockerfile:
FROM ubuntu
ARG CONT_IMG_VER
ENV CONT_IMG_VER=hello
RUN echo $CONT_IMG_VER
Line 3 doesn't cause a cache miss because the value of CONT_IMG_VER
is a
constant (hello
). As a result, the environment variables and values used on
the RUN
(line 4) doesn't change between builds.
ONBUILD
ONBUILD <INSTRUCTION>
The ONBUILD
instruction adds to the image a trigger instruction to
be executed at a later time, when the image is used as the base for
another build. The trigger will be executed in the context of the
downstream build, as if it had been inserted immediately after the
FROM
instruction in the downstream Dockerfile.
Any build instruction can be registered as a trigger.
This is useful if you are building an image which will be used as a base to build other images, for example an application build environment or a daemon which may be customized with user-specific configuration.
For example, if your image is a reusable Python application builder, it
will require application source code to be added in a particular
directory, and it might require a build script to be called after
that. You can't just call ADD
and RUN
now, because you don't yet
have access to the application source code, and it will be different for
each application build. You could simply provide application developers
with a boilerplate Dockerfile to copy-paste into their application, but
that's inefficient, error-prone and difficult to update because it
mixes with application-specific code.
The solution is to use ONBUILD
to register advance instructions to
run later, during the next build stage.
Here's how it works:
- When it encounters an
ONBUILD
instruction, the builder adds a trigger to the metadata of the image being built. The instruction doesn't otherwise affect the current build. - At the end of the build, a list of all triggers is stored in the
image manifest, under the key
OnBuild
. They can be inspected with thedocker inspect
command. - Later the image may be used as a base for a new build, using the
FROM
instruction. As part of processing theFROM
instruction, the downstream builder looks forONBUILD
triggers, and executes them in the same order they were registered. If any of the triggers fail, theFROM
instruction is aborted which in turn causes the build to fail. If all triggers succeed, theFROM
instruction completes and the build continues as usual. - Triggers are cleared from the final image after being executed. In other words they aren't inherited by "grand-children" builds.
For example you might add something like this:
ONBUILD ADD . /app/src
ONBUILD RUN /usr/local/bin/python-build --dir /app/src
ONBUILD limitations
- Chaining
ONBUILD
instructions usingONBUILD ONBUILD
isn't allowed. - The
ONBUILD
instruction may not triggerFROM
orMAINTAINER
instructions. ONBUILD COPY --from
is not supported.
STOPSIGNAL
STOPSIGNAL signal
The STOPSIGNAL
instruction sets the system call signal that will be sent to the
container to exit. This signal can be a signal name in the format SIG<NAME>
,
for instance SIGKILL
, or an unsigned number that matches a position in the
kernel's syscall table, for instance 9
. The default is SIGTERM
if not
defined.
The image's default stopsignal can be overridden per container, using the
--stop-signal
flag on docker run
and docker create
.
HEALTHCHECK
The HEALTHCHECK
instruction has two forms:
HEALTHCHECK [OPTIONS] CMD command
(check container health by running a command inside the container)HEALTHCHECK NONE
(disable any healthcheck inherited from the base image)
The HEALTHCHECK
instruction tells Docker how to test a container to check that
it's still working. This can detect cases such as a web server stuck in
an infinite loop and unable to handle new connections, even though the server
process is still running.
When a container has a healthcheck specified, it has a health status in
addition to its normal status. This status is initially starting
. Whenever a
health check passes, it becomes healthy
(whatever state it was previously in).
After a certain number of consecutive failures, it becomes unhealthy
.
The options that can appear before CMD
are:
--interval=DURATION
(default:30s
)--timeout=DURATION
(default:30s
)--start-period=DURATION
(default:0s
)--start-interval=DURATION
(default:5s
)--retries=N
(default:3
)
The health check will first run interval seconds after the container is started, and then again interval seconds after each previous check completes.
If a single run of the check takes longer than timeout seconds then the check is considered to have failed.
It takes retries consecutive failures of the health check for the container
to be considered unhealthy
.
start period provides initialization time for containers that need time to bootstrap. Probe failure during that period will not be counted towards the maximum number of retries. However, if a health check succeeds during the start period, the container is considered started and all consecutive failures will be counted towards the maximum number of retries.
start interval is the time between health checks during the start period. This option requires Docker Engine version 25.0 or later.
There can only be one HEALTHCHECK
instruction in a Dockerfile. If you list
more than one then only the last HEALTHCHECK
will take effect.
The command after the CMD
keyword can be either a shell command (e.g. HEALTHCHECK CMD /bin/check-running
) or an exec array (as with other Dockerfile commands;
see e.g. ENTRYPOINT
for details).
The command's exit status indicates the health status of the container. The possible values are:
- 0: success - the container is healthy and ready for use
- 1: unhealthy - the container isn't working correctly
- 2: reserved - don't use this exit code
For example, to check every five minutes or so that a web-server is able to serve the site's main page within three seconds:
HEALTHCHECK --interval=5m --timeout=3s \
CMD curl -f http://localhost/ || exit 1
To help debug failing probes, any output text (UTF-8 encoded) that the command writes
on stdout or stderr will be stored in the health status and can be queried with
docker inspect
. Such output should be kept short (only the first 4096 bytes
are stored currently).
When the health status of a container changes, a health_status
event is
generated with the new status.
SHELL
SHELL ["executable", "parameters"]
The SHELL
instruction allows the default shell used for the shell form of
commands to be overridden. The default shell on Linux is ["/bin/sh", "-c"]
, and on
Windows is ["cmd", "/S", "/C"]
. The SHELL
instruction must be written in JSON
form in a Dockerfile.
The SHELL
instruction is particularly useful on Windows where there are
two commonly used and quite different native shells: cmd
and powershell
, as
well as alternate shells available including sh
.
The SHELL
instruction can appear multiple times. Each SHELL
instruction overrides
all previous SHELL
instructions, and affects all subsequent instructions. For example:
FROM microsoft/windowsservercore
# Executed as cmd /S /C echo default
RUN echo default
# Executed as cmd /S /C powershell -command Write-Host default
RUN powershell -command Write-Host default
# Executed as powershell -command Write-Host hello
SHELL ["powershell", "-command"]
RUN Write-Host hello
# Executed as cmd /S /C echo hello
SHELL ["cmd", "/S", "/C"]
RUN echo hello
The following instructions can be affected by the SHELL
instruction when the
shell form of them is used in a Dockerfile: RUN
, CMD
and ENTRYPOINT
.
The following example is a common pattern found on Windows which can be
streamlined by using the SHELL
instruction:
RUN powershell -command Execute-MyCmdlet -param1 "c:\foo.txt"
The command invoked by the builder will be:
cmd /S /C powershell -command Execute-MyCmdlet -param1 "c:\foo.txt"
This is inefficient for two reasons. First, there is an unnecessary cmd.exe
command processor (aka shell) being invoked. Second, each RUN
instruction in
the shell form requires an extra powershell -command
prefixing the command.
To make this more efficient, one of two mechanisms can be employed. One is to
use the JSON form of the RUN
command such as:
RUN ["powershell", "-command", "Execute-MyCmdlet", "-param1 \"c:\\foo.txt\""]
While the JSON form is unambiguous and does not use the unnecessary cmd.exe
,
it does require more verbosity through double-quoting and escaping. The alternate
mechanism is to use the SHELL
instruction and the shell form,
making a more natural syntax for Windows users, especially when combined with
the escape
parser directive:
# escape=`
FROM microsoft/nanoserver
SHELL ["powershell","-command"]
RUN New-Item -ItemType Directory C:\Example
ADD Execute-MyCmdlet.ps1 c:\example\
RUN c:\example\Execute-MyCmdlet -sample 'hello world'
Resulting in:
PS E:\myproject> docker build -t shell .
Sending build context to Docker daemon 4.096 kB
Step 1/5 : FROM microsoft/nanoserver
---> 22738ff49c6d
Step 2/5 : SHELL powershell -command
---> Running in 6fcdb6855ae2
---> 6331462d4300
Removing intermediate container 6fcdb6855ae2
Step 3/5 : RUN New-Item -ItemType Directory C:\Example
---> Running in d0eef8386e97
Directory: C:\
Mode LastWriteTime Length Name
---- ------------- ------ ----
d----- 10/28/2016 11:26 AM Example
---> 3f2fbf1395d9
Removing intermediate container d0eef8386e97
Step 4/5 : ADD Execute-MyCmdlet.ps1 c:\example\
---> a955b2621c31
Removing intermediate container b825593d39fc
Step 5/5 : RUN c:\example\Execute-MyCmdlet 'hello world'
---> Running in be6d8e63fe75
hello world
---> 8e559e9bf424
Removing intermediate container be6d8e63fe75
Successfully built 8e559e9bf424
PS E:\myproject>
The SHELL
instruction could also be used to modify the way in which
a shell operates. For example, using SHELL cmd /S /C /V:ON|OFF
on Windows, delayed
environment variable expansion semantics could be modified.
The SHELL
instruction can also be used on Linux should an alternate shell be
required such as zsh
, csh
, tcsh
and others.
Here-Documents
Here-documents allow redirection of subsequent Dockerfile lines to the input of
RUN
or COPY
commands. If such command contains a
here-document
the Dockerfile considers the next lines until the line only containing a
here-doc delimiter as part of the same command.
Example: Running a multi-line script
# syntax=docker/dockerfile:1
FROM debian
RUN <<EOT bash
set -ex
apt-get update
apt-get install -y vim
EOT
If the command only contains a here-document, its contents is evaluated with the default shell.
# syntax=docker/dockerfile:1
FROM debian
RUN <<EOT
mkdir -p foo/bar
EOT
Alternatively, shebang header can be used to define an interpreter.
# syntax=docker/dockerfile:1
FROM python:3.6
RUN <<EOT
#!/usr/bin/env python
print("hello world")
EOT
More complex examples may use multiple here-documents.
# syntax=docker/dockerfile:1
FROM alpine
RUN <<FILE1 cat > file1 && <<FILE2 cat > file2
I am
first
FILE1
I am
second
FILE2
Example: Creating inline files
With COPY
instructions, you can replace the source parameter with a here-doc
indicator to write the contents of the here-document directly to a file. The
following example creates a greeting.txt
file containing hello world
using
a COPY
instruction.
# syntax=docker/dockerfile:1
FROM alpine
COPY <<EOF greeting.txt
hello world
EOF
Regular here-doc
variable expansion and tab stripping rules apply.
The following example shows a small Dockerfile that creates a hello.sh
script
file using a COPY
instruction with a here-document.
# syntax=docker/dockerfile:1
FROM alpine
ARG FOO=bar
COPY <<-EOT /script.sh
echo "hello ${FOO}"
EOT
ENTRYPOINT ash /script.sh
In this case, file script prints "hello bar", because the variable is expanded
when the COPY
instruction gets executed.
$ docker build -t heredoc .
$ docker run heredoc
hello bar
If instead you were to quote any part of the here-document word EOT
, the
variable would not be expanded at build-time.
# syntax=docker/dockerfile:1
FROM alpine
ARG FOO=bar
COPY <<-"EOT" /script.sh
echo "hello ${FOO}"
EOT
ENTRYPOINT ash /script.sh
Note that ARG FOO=bar
is excessive here, and can be removed. The variable
gets interpreted at runtime, when the script is invoked:
$ docker build -t heredoc .
$ docker run -e FOO=world heredoc
hello world
Dockerfile examples
For examples of Dockerfiles, refer to: