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.

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):

$ 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.

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)
  • 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 (').

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.

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

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 (').

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).

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 will copy new files from <src> and add them to the container's filesystem at path <dest>.

<src> must be the path to a file or directory relative to the source directory being built (also called the context of the build) or a remote file URL.

<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.

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 <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 will copy new files from <src> and add them to the container's filesystem at path <dest>.

<src> must be the path to a file or directory relative to the source directory being built (also called the context of the build).

<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 <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"] (like an exec, the preferred form)
  • ENTRYPOINT command param1 param2 (as a shell)

There can only be one ENTRYPOINT in a Dockerfile. If you have more than one ENTRYPOINT, then only the last one in the Dockerfile will have an effect.

An ENTRYPOINT helps you to configure a container that you can run as an executable. That is, when you specify an ENTRYPOINT, then the whole container runs as if it was just that executable.

Unlike the behavior of the CMD instruction, The ENTRYPOINT instruction adds an entry command that will not be overwritten when arguments are passed to docker run. 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 specify parameters either in the ENTRYPOINT JSON array (as in "like an exec" above), or by using a CMD instruction. Parameters in the ENTRYPOINT instruction will not be overridden by the docker run arguments, but parameters specified via a CMD instruction will be overridden by docker run arguments.

Like a CMD, you can specify a plain string for the ENTRYPOINT and it will execute in /bin/sh -c:

FROM ubuntu
ENTRYPOINT ls -l

For example, that Dockerfile's image will always take a directory as an input and return a directory listing. If you wanted to make this optional but default, you could use a CMD instruction:

FROM ubuntu
CMD ["-l"]
ENTRYPOINT ["ls"]

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

Note: It is preferable to use the JSON array format for specifying ENTRYPOINT instructions.

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, VOLUME /var/log. 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 following RUN directives.

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.

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.