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 you would normally execute manually in order to build a Docker image. By calling docker build from your terminal, you can have Docker build your image step by step, executing the instructions successively.

This page discusses the specifics of all the instructions you can use in your Dockerfile. To further help you write a clear, readable, maintainable Dockerfile, we've also written a Dockerfile Best Practices guide. Lastly, you can test your Dockerfile knowledge with the Dockerfile tutorial.

Usage

To build an image from a source repository, create a description file called Dockerfile at the root of your repository. This file will describe the steps to assemble the image.

Then call docker build with the path of your source repository as the argument (for example, .):

$ sudo docker build .

The path to the source repository defines where to find the context of the build. The build is run by the Docker daemon, not by the CLI, so the whole context must be transferred to the daemon. The Docker CLI reports "Sending build context to Docker daemon" when the context is sent to the daemon.

Warning Avoid using your root directory, /, as the root of the source repository. The docker build command will use whatever directory contains the Dockerfile as the build context (including all of its subdirectories). The build context will be sent to the Docker daemon before building the image, which means if you use / as the source repository, the entire contents of your hard drive will get sent to the daemon (and thus to the machine running the daemon). You probably don't want that.

In most cases, it's best to put each Dockerfile in an empty directory, and then add only the files needed for building that Dockerfile to that directory. To further speed up the build, you can exclude files and directories by adding a .dockerignore file to the same directory.

You can specify a repository and tag at which to save the new image if the build succeeds:

$ sudo docker build -t shykes/myapp .

The Docker daemon will run your steps one-by-one, committing the result to a new image if necessary, before finally outputting the ID of your new image. The Docker daemon will automatically clean up the context you sent.

Note that each instruction is run independently, and causes a new image to be created - so RUN cd /tmp will not have any effect on the next instructions.

Whenever possible, Docker will re-use the intermediate images, accelerating docker build significantly (indicated by Using cache - see the Dockerfile Best Practices guide for more information):

$ sudo docker build -t SvenDowideit/ambassador .
Uploading context 10.24 kB
Uploading context
Step 1 : FROM docker-ut
 ---> cbba202fe96b
Step 2 : MAINTAINER SvenDowideit@home.org.au
 ---> Using cache
 ---> 51182097be13
Step 3 : CMD env | grep _TCP= | sed 's/.*_PORT_\([0-9]*\)_TCP=tcp:\/\/\(.*\):\(.*\)/socat TCP4-LISTEN:\1,fork,reuseaddr TCP4:\2:\3 \&/'  | sh && top
 ---> Using cache
 ---> 1a5ffc17324d
Successfully built 1a5ffc17324d

When you're done with your build, you're ready to look into Pushing a repository to its registry.

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 in order to distinguish them from arguments more easily.

Docker runs the instructions in a Dockerfile in order. The first instruction must be `FROM` in order to specify the Base Image from which you are building.

Docker will treat lines that begin with # as a comment. A # marker anywhere else in the line will be treated as an argument. This allows statements like:

# Comment
RUN echo 'we are running some # of cool things'

Here is the set of instructions you can use in a Dockerfile for building images.

Environment Replacement

Note: prior to 1.3, Dockerfile environment variables were handled similarly, in that they would be replaced as described below. However, there was no formal definition on as to which instructions handled environment replacement at the time. After 1.3 this behavior will be preserved and canonical.

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.

Escaping is possible 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

The instructions that handle environment variables in the Dockerfile are:

  • ENV
  • ADD
  • COPY
  • WORKDIR
  • EXPOSE
  • VOLUME
  • USER

ONBUILD instructions are NOT supported for environment replacement, even the instructions above.

The .dockerignore file

If a file named .dockerignore exists in the source repository, then it is interpreted as a newline-separated list of exclusion patterns. Exclusion patterns match files or directories relative to the source repository that will be excluded from the context. Globbing is done using Go's filepath.Match rules.

The following example shows the use of the .dockerignore file to exclude the .git directory from the context. Its effect can be seen in the changed size of the uploaded context.

$ sudo docker build .
Uploading context 18.829 MB
Uploading context
Step 0 : FROM busybox
 ---> 769b9341d937
Step 1 : CMD echo Hello World
 ---> Using cache
 ---> 99cc1ad10469
Successfully built 99cc1ad10469
$ echo ".git" > .dockerignore
$ sudo docker build .
Uploading context  6.76 MB
Uploading context
Step 0 : FROM busybox
 ---> 769b9341d937
Step 1 : CMD echo Hello World
 ---> Using cache
 ---> 99cc1ad10469
Successfully built 99cc1ad10469

FROM

FROM <image>

Or

FROM <image>:<tag>

The FROM instruction sets the Base Image for subsequent instructions. As such, a valid Dockerfile must have FROM as its first instruction. The image can be any valid image – it is especially easy to start by pulling an image from the Public Repositories.

FROM must be the first non-comment instruction in the Dockerfile.

FROM can appear multiple times within a single Dockerfile in order to create multiple images. Simply make a note of the last image ID output by the commit before each new FROM command.

If no tag is given to the FROM instruction, latest is assumed. If the used tag does not exist, an error will be returned.

MAINTAINER

MAINTAINER <name>

The MAINTAINER instruction allows you to set the Author field of the generated images.

RUN

RUN has 2 forms:

  • RUN <command> (the command is run in a shell - /bin/sh -c - shell form)
  • RUN ["executable", "param1", "param2"] (exec form)

The RUN instruction will execute any commands in a new layer on top of the current image and commit the results. The resulting committed image will be used for the next step in the Dockerfile.

Layering RUN instructions and generating commits conforms to the core concepts of Docker where commits are cheap and containers can be created from any point in an image's history, much like source control.

The exec form makes it possible to avoid shell string munging, and to RUN commands using a base image that does not contain /bin/sh.

Note: To use a different shell, other than '/bin/sh', use the exec form passing in the desired shell. For example, RUN ["/bin/bash", "-c", "echo hello"]

Note: The exec form is parsed as a JSON array, which means that you must use double-quotes (") around words not single-quotes (').

Note: Unlike the shell form, the exec form does not invoke a command shell. This means that normal shell processing does not happen. For example, RUN [ "echo", "$HOME" ] will not do variable substitution on $HOME. If you want shell processing then either use the shell form or execute a shell directly, for example: RUN [ "sh", "-c", "echo", "$HOME" ].

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 instructions. See below for details.

Known Issues (RUN)

  • Issue 783 is about file permissions problems that can occur when using the AUFS file system. You might notice it during an attempt to rm a file, for example. The issue describes a workaround.

CMD

The CMD instruction has three forms:

  • CMD ["executable","param1","param2"] (exec form, this is the preferred form)
  • CMD ["param1","param2"] (as default parameters to ENTRYPOINT)
  • CMD command param1 param2 (shell form)

There can only be one CMD instruction in a Dockerfile. If you list more than one CMD then only the last CMD will take effect.

The main 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.

Note: If CMD is used to provide default arguments for the ENTRYPOINT instruction, both the CMD and ENTRYPOINT instructions should be specified with the JSON array format.

Note: The exec form is parsed as a JSON array, which means that you must use double-quotes (") around words not single-quotes (').

Note: Unlike the shell form, the exec form does not invoke a command shell. This means that normal shell processing does not happen. For example, CMD [ "echo", "$HOME" ] will not do variable substitution on $HOME. If you want shell processing then either use the shell form or execute a shell directly, for example: CMD [ "sh", "-c", "echo", "$HOME" ].

When used in the shell or exec formats, the CMD instruction sets the command to be executed when running the image.

If you use the shell form of the CMD, then the <command> will execute in /bin/sh -c:

FROM ubuntu
CMD echo "This is a test." | wc -

If you want to run your <command> without a shell then you must express the command as a JSON array and give the full path to the executable. This array form is the preferred format of CMD. Any additional parameters must be individually expressed as strings in the array:

FROM ubuntu
CMD ["/usr/bin/wc","--help"]

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.

Note: don't confuse RUN with CMD. RUN actually runs a command and commits the result; CMD does not execute anything at build time, but specifies the intended command for the image.

EXPOSE

EXPOSE <port> [<port>...]

The EXPOSE instructions informs Docker that the container will listen on the specified network ports at runtime. Docker uses this information to interconnect containers using links (see the Docker User Guide). Note that EXPOSE only works for inter-container links. It doesn't make ports accessible from the host. To expose ports to the host, at runtime, use the -p flag.

ENV

ENV <key> <value>

The ENV instruction sets the environment variable <key> to the value <value>. This value will be passed to all future RUN instructions. This is functionally equivalent to prefixing the command with <key>=<value>

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>.

Note: One example where this can cause unexpected consequences, is setting ENV DEBIAN_FRONTEND noninteractive. Which will persist when the container is run interactively; for example: docker run -t -i image bash

ADD

ADD <src>... <dest>

The ADD instruction copies new files, directories or remote file URLs from <src> and adds them to the filesystem of the container at the path <dest>.

Multiple <src> resource may be specified but if they are files or directories then they must be relative to the source directory that is being built (the context of the build).

Each <src> may contain wildcards and matching will be done using Go's filepath.Match rules. For most command line uses this should act as expected, for example:

ADD hom* /mydir/        # adds all files starting with "hom"
ADD hom?.txt /mydir/    # ? is replaced with any single character

The <dest> is the absolute path to which the source will be copied inside the destination container.

All new files and directories are created with a UID and GID of 0.

In the case where <src> is a remote file URL, the destination will have permissions of 600.

Note: If you build by passing a Dockerfile through STDIN (docker build - < somefile), there is no build context, so the Dockerfile can only contain a URL based ADD instruction. You can also pass a compressed archive through STDIN: (docker build - < archive.tar.gz), the Dockerfile at the root of the archive and the rest of the archive will get used at the context of the build.

Note: If your URL files are protected using authentication, you will need to use RUN wget, RUN curl or use another tool from within the container as the ADD instruction does not support authentication.

Note: The first encountered ADD instruction will invalidate the cache for all following instructions from the Dockerfile if the contents of <src> have changed. This includes invalidating the cache for RUN instructions. See the Dockerfile Best Practices guide for more information.

The copy obeys the following rules:

  • The <src> path must be inside the context of the build; you cannot ADD ../something /something, because the first step of a docker build is to send the context directory (and subdirectories) to the docker daemon.

  • If <src> is a URL and <dest> does not end with a trailing slash, then a file is downloaded from the URL and copied to <dest>.

  • If <src> is a URL and <dest> does end with a trailing slash, then the filename is inferred from the URL and the file is downloaded to <dest>/<filename>. For instance, 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 in this case (http://example.com will not work).

  • If <src> is a directory, the entire directory is copied, including filesystem metadata.

  • If <src> is a local tar archive in a recognized compression format (identity, gzip, bzip2 or xz) then it is unpacked as a directory. Resources from remote URLs are not decompressed. When a directory is copied or unpacked, it has the same behavior as tar -x: the result is the union of:

    1. Whatever existed at the destination path and
    2. The contents of the source tree, with conflicts resolved in favor of "2." on a file-by-file basis.
  • If <src> is any other kind of file, it is copied individually along with its metadata. In this case, if <dest> ends with a trailing slash /, it will be considered a directory and the contents of <src> will be written at <dest>/base(<src>).

  • If multiple <src> resources are specified, either directly or due to the use of a wildcard, then <dest> must be a directory, and it must end with a slash /.

  • If <dest> does not end with a trailing slash, it will be considered a regular file and the contents of <src> will be written at <dest>.

  • If <dest> doesn't exist, it is created along with all missing directories in its path.

COPY

COPY <src>... <dest>

The COPY instruction copies new files or directories from <src> and adds them to the filesystem of the container at the path <dest>.

Multiple <src> resource may be specified but they must be relative to the source directory that is being built (the context of the build).

Each <src> may contain wildcards and matching will be done using Go's filepath.Match rules. For most command line uses this should act as expected, for example:

COPY hom* /mydir/        # adds all files starting with "hom"
COPY hom?.txt /mydir/    # ? is replaced with any single character

The <dest> is the absolute path to which the source will be copied inside the destination container.

All new files and directories are created with a UID and GID of 0.

Note: If you build using STDIN (docker build - < somefile), there is no build context, so COPY can't be used.

The copy obeys the following rules:

  • The <src> path must be inside the context of the build; you cannot COPY ../something /something, because the first step of a docker build is to send the context directory (and subdirectories) to the docker daemon.

  • If <src> is a directory, the entire directory is copied, including filesystem metadata.

  • If <src> is any other kind of file, it is copied individually along with its metadata. In this case, if <dest> ends with a trailing slash /, it will be considered a directory and the contents of <src> will be written at <dest>/base(<src>).

  • If multiple <src> resources are specified, either directly or due to the use of a wildcard, then <dest> must be a directory, and it must end with a slash /.

  • If <dest> does not end with a trailing slash, it will be considered a regular file and the contents of <src> will be written at <dest>.

  • If <dest> doesn't exist, it is created along with all missing directories in its path.

ENTRYPOINT

ENTRYPOINT has two forms:

  • ENTRYPOINT ["executable", "param1", "param2"] (the preferred exec form)
  • ENTRYPOINT command param1 param2 (shell form)

An ENTRYPOINT allows you to configure a container that will run as an executable.

For example, the following will start 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 prevents any CMD or run command line arguments from being used, but has the disadvantage that your ENTRYPOINT will be started 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 - so your executable will not 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.

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 (see the Dockerfile best practices for more details):

#!/bin/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 KILL 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 over ride the ENTRYPOINT setting using --entrypoint, but this can only set the binary to exec (no sh -c will be used).

Note: The exec form is parsed as a JSON array, which means that you must use double-quotes (") around words not single-quotes (').

Note: Unlike the shell form, the exec form does not invoke a command shell. This means that normal shell processing does not happen. For example, ENTRYPOINT [ "echo", "$HOME" ] will not do variable substitution on $HOME. If you want shell processing then either use the shell form or execute a shell directly, for example: ENTRYPOINT [ "sh", "-c", "echo", "$HOME" ]. Variables that are defined in the Dockerfileusing ENV, will be substituted by the Dockerfile parser.

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 will exit 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
Mem: 1704184K used, 352484K free, 0K shrd, 0K buff, 140621524238337K cached
CPU:   9% usr   2% sys   0% nic  88% idle   0% io   0% irq   0% sirq
Load average: 0.01 0.02 0.05 2/101 7
  PID  PPID USER     STAT   VSZ %VSZ %CPU COMMAND
    1     0 root     S     3168   0%   0% /bin/sh -c top -b cmd cmd2
    7     1 root     R     3164   0%   0% top -b

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 aux
PID   USER     COMMAND
    1 root     /bin/sh -c top -b cmd cmd2
    7 root     top -b
    8 root     ps aux
$ /usr/bin/time docker stop test
test
real    0m 10.19s
user    0m 0.04s
sys 0m 0.03s

VOLUME

VOLUME ["/data"]

The VOLUME instruction will create a mount point with the specified name and mark 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.

Note: The list is parsed a JSON array, which means that you must use double-quotes (") around words not single-quotes (').

USER

USER daemon

The USER instruction sets the user name or UID to use when running the image and for any RUN, CMD and ENTRYPOINT instructions that follow it in the Dockerfile.

WORKDIR

WORKDIR /path/to/workdir

The WORKDIR instruction sets the working directory for any RUN, CMD and ENTRYPOINT instructions that follow it in the Dockerfile.

It can be used multiple times in the one 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

The output of the final pwd command in this Dockerfile would be /path/$DIRNAME

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 is 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:

  1. When it encounters an ONBUILD instruction, the builder adds a trigger to the metadata of the image being built. The instruction does not otherwise affect the current build.
  2. 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 the docker inspect command.
  3. Later the image may be used as a base for a new build, using the FROM instruction. As part of processing the FROM instruction, the downstream builder looks for ONBUILD triggers, and executes them in the same order they were registered. If any of the triggers fail, the FROM instruction is aborted which in turn causes the build to fail. If all triggers succeed, the FROM instruction completes and the build continues as usual.
  4. Triggers are cleared from the final image after being executed. In other words they are not 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
[...]

Warning: Chaining ONBUILD instructions using ONBUILD ONBUILD isn't allowed.

Warning: The ONBUILD instruction may not trigger FROM or MAINTAINER instructions.

Dockerfile Examples

# Nginx
#
# VERSION               0.0.1

FROM      ubuntu
MAINTAINER Victor Vieux <victor@docker.com>

RUN apt-get update && apt-get install -y inotify-tools nginx apache2 openssh-server

# Firefox over VNC
#
# VERSION               0.3

FROM ubuntu

# Install vnc, xvfb in order to create a 'fake' display and firefox
RUN apt-get update && apt-get install -y x11vnc xvfb firefox
RUN mkdir ~/.vnc
# Setup a password
RUN x11vnc -storepasswd 1234 ~/.vnc/passwd
# Autostart firefox (might not be the best way, but it does the trick)
RUN bash -c 'echo "firefox" >> /.bashrc'

EXPOSE 5900
CMD    ["x11vnc", "-forever", "-usepw", "-create"]

# Multiple images example
#
# VERSION               0.1

FROM ubuntu
RUN echo foo > bar
# Will output something like ===> 907ad6c2736f

FROM ubuntu
RUN echo moo > oink
# Will output something like ===> 695d7793cbe4

# You᾿ll now have two images, 907ad6c2736f with /bar, and 695d7793cbe4 with
# /oink.