moby/docs/sources/reference/builder.rst

title

Build Images (Dockerfile Reference)

description

Dockerfiles use a simple DSL which allows you to automate the steps you would normally manually take to create an image.

keywords

builder, docker, Dockerfile, automation, image creation

Build Images (Dockerfile Reference)

Docker can act as a builder and read instructions from a text Dockerfile to automate the steps you would otherwise take manually to create an image. Executing docker build will run your steps and commit them along the way, giving you a final image.

Table of Contents

1. Usage

To build <cli_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 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 "Uploading context" when the context is sent to the daemon.

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:

$ 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 image_push.

2. Format

The Dockerfile format is quite simple:

# 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 evaluates the instructions in a Dockerfile in order. The first instruction must be `FROM` in order to specify the base_image_def 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'

3. Instructions

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

3.1 FROM

FROM <image>

Or

FROM <image>:<tag>

The FROM instruction sets the base_image_def 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 using_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.

3.2 MAINTAINER

MAINTAINER <name>

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

3.3 RUN

RUN <command>

The RUN instruction will execute any commands on 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.

Known Issues (RUN)

• 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.
• 2424 Locale will not be set automatically.

3.4 CMD

CMD has three forms:

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

There can only be one CMD 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 as well.

When used in the shell or exec formats, the CMD instruction sets the command to be executed when running the image. This is functionally equivalent to running docker commit -run '{"Cmd": <command>}' outside the builder.

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 dockerfile_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.

3.5 EXPOSE

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

The EXPOSE instruction exposes ports for use within links. This is functionally equivalent to running docker commit -run '{"PortSpecs": ["<port>", "<port2>"]}' outside the builder. Refer to port_redirection for detailed information.

3.6 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>

Note

The environment variables will persist when a container is run from the resulting image.

3.7 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 path at which the source will be copied in the destination container.

All new files and directories are created with mode 0755, uid and gid 0.

Note

if you build using STDIN (docker build - < somefile), there is no build context, so the Dockerfile can only contain an URL based ADD statement.

Note

if your URL files are protected using authentication, you will need to use an RUN wget , RUN curl or other tool from within the container as ADD does not support authentication.

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.

3.8 ENTRYPOINT

ENTRYPOINT has two forms:

• ENTRYPOINT ["executable", "param1", "param2"] (like an exec, 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.

The ENTRYPOINT instruction adds an entry command that will not be overwritten when arguments are passed to docker run, unlike the behavior of CMD. This allows arguments to be passed to the entrypoint. i.e. docker run <image> -d will pass the "-d" argument to the ENTRYPOINT.

You can specify parameters either in the ENTRYPOINT JSON array (as in "like an exec" above), or by using a CMD statement. Parameters in the ENTRYPOINT will not be overridden by the docker run arguments, but parameters specified via CMD 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 wc -l -

For example, that Dockerfile's image will always take stdin as input ("-") and print the number of lines ("-l"). If you wanted to make this optional but default, you could use a CMD:

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

3.9 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. For more information/examples and mounting instructions via docker client, refer to volume_def documentation.

3.10 USER

USER daemon

The USER instruction sets the username or UID to use when running the image.

3.11 WORKDIR

WORKDIR /path/to/workdir

The WORKDIR instruction sets the working directory in which the command given by CMD is executed.

4. Dockerfile Examples

# Nginx
#
# VERSION               0.0.1

FROM      ubuntu
MAINTAINER Guillaume J. Charmes <guillaume@dotcloud.com>

# make sure the package repository is up to date
RUN echo "deb http://archive.ubuntu.com/ubuntu precise main universe" > /etc/apt/sources.list
RUN apt-get update

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

# Firefox over VNC
#
# VERSION               0.3

FROM ubuntu
# make sure the package repository is up to date
RUN echo "deb http://archive.ubuntu.com/ubuntu precise main universe" > /etc/apt/sources.list
RUN apt-get update

# Install vnc, xvfb in order to create a 'fake' display and firefox
RUN 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.