docker build

Description

Build an image from a Dockerfile

Usage

docker build [OPTIONS] PATH | URL | -

Options

Parent command

Extended description

The docker build command builds Docker images from a Dockerfile and a “context”. A build’s context is the set of files located in the specified PATH or URL. The build process can refer to any of the files in the context. For example, your build can use a COPY instruction to reference a file in the context.

The URL parameter can refer to three kinds of resources: Git repositories, pre-packaged tarball contexts and plain text files.

Git repositories

When the URL parameter points to the location of a Git repository, the repository acts as the build context. The system recursively fetches the repository and its submodules. The commit history is not preserved. A repository is first pulled into a temporary directory on your local host. After that succeeds, the directory is sent to the Docker daemon as the context. Local copy gives you the ability to access private repositories using local user credentials, VPN’s, and so forth.

Note: If the URL parameter contains a fragment the system will recursively clone the repository and its submodules using a git clone --recursive command.

Git URLs accept context configuration in their fragment section, separated by a colon :. The first part represents the reference that Git will check out, and can be either a branch, a tag, or a remote reference. The second part represents a subdirectory inside the repository that will be used as a build context.

For example, run this command to use a directory called docker in the branch container:

$ docker build https://github.com/docker/rootfs.git#container:docker

The following table represents all the valid suffixes with their build contexts:

Tarball contexts

If you pass an URL to a remote tarball, the URL itself is sent to the daemon:

$ docker build http://server/context.tar.gz

The download operation will be performed on the host the Docker daemon is running on, which is not necessarily the same host from which the build command is being issued. The Docker daemon will fetch context.tar.gz and use it as the build context. Tarball contexts must be tar archives conforming to the standard tar UNIX format and can be compressed with any one of the ‘xz’, ‘bzip2’, ‘gzip’ or ‘identity’ (no compression) formats.

Text files

Instead of specifying a context, you can pass a single Dockerfile in the URL or pipe the file in via STDIN. To pipe a Dockerfile from STDIN:

$ docker build - < Dockerfile

With Powershell on Windows, you can run:

Get-Content Dockerfile | docker build -

If you use STDIN or specify a URL pointing to a plain text file, the system places the contents into a file called Dockerfile, and any -f, --file option is ignored. In this scenario, there is no context.

By default the docker build command will look for a Dockerfile at the root of the build context. The -f, --file, option lets you specify the path to an alternative file to use instead. This is useful in cases where the same set of files are used for multiple builds. The path must be to a file within the build context. If a relative path is specified then it is interpreted as relative to the root of the context.

In most cases, it’s best to put each Dockerfile in an empty directory. Then, add to that directory only the files needed for building the Dockerfile. To increase the build’s performance, you can exclude files and directories by adding a .dockerignore file to that directory as well. For information on creating one, see the .dockerignore file.

If the Docker client loses connection to the daemon, the build is canceled. This happens if you interrupt the Docker client with CTRL-c or if the Docker client is killed for any reason. If the build initiated a pull which is still running at the time the build is cancelled, the pull is cancelled as well.

Examples

Build with PATH

$ docker build .

Uploading context 10240 bytes
Step 1/3 : FROM busybox
Pulling repository busybox
 ---> e9aa60c60128MB/2.284 MB (100%) endpoint: https://cdn-registry-1.docker.io/v1/
Step 2/3 : RUN ls -lh /
 ---> Running in 9c9e81692ae9
total 24
drwxr-xr-x    2 root     root        4.0K Mar 12  2013 bin
drwxr-xr-x    5 root     root        4.0K Oct 19 00:19 dev
drwxr-xr-x    2 root     root        4.0K Oct 19 00:19 etc
drwxr-xr-x    2 root     root        4.0K Nov 15 23:34 lib
lrwxrwxrwx    1 root     root           3 Mar 12  2013 lib64 -> lib
dr-xr-xr-x  116 root     root           0 Nov 15 23:34 proc
lrwxrwxrwx    1 root     root           3 Mar 12  2013 sbin -> bin
dr-xr-xr-x   13 root     root           0 Nov 15 23:34 sys
drwxr-xr-x    2 root     root        4.0K Mar 12  2013 tmp
drwxr-xr-x    2 root     root        4.0K Nov 15 23:34 usr
 ---> b35f4035db3f
Step 3/3 : CMD echo Hello world
 ---> Running in 02071fceb21b
 ---> f52f38b7823e
Successfully built f52f38b7823e
Removing intermediate container 9c9e81692ae9
Removing intermediate container 02071fceb21b

This example specifies that the PATH is ., and so all the files in the local directory get tard and sent to the Docker daemon. The PATH specifies where to find the files for the “context” of the build on the Docker daemon. Remember that the daemon could be running on a remote machine and that no parsing of the Dockerfile happens at the client side (where you’re running docker build). That means that all the files at PATH get sent, not just the ones listed to ADD in the Dockerfile.

The transfer of context from the local machine to the Docker daemon is what the docker client means when you see the “Sending build context” message.

If you wish to keep the intermediate containers after the build is complete, you must use --rm=false. This does not affect the build cache.

Build with URL

$ docker build github.com/creack/docker-firefox

This will clone the GitHub repository and use the cloned repository as context. The Dockerfile at the root of the repository is used as Dockerfile. You can specify an arbitrary Git repository by using the git:// or git@ scheme.

$ docker build -f ctx/Dockerfile http://server/ctx.tar.gz

Downloading context: http://server/ctx.tar.gz [===================>]    240 B/240 B
Step 1/3 : FROM busybox
 ---> 8c2e06607696
Step 2/3 : ADD ctx/container.cfg /
 ---> e7829950cee3
Removing intermediate container b35224abf821
Step 3/3 : CMD /bin/ls
 ---> Running in fbc63d321d73
 ---> 3286931702ad
Removing intermediate container fbc63d321d73
Successfully built 377c409b35e4

This sends the URL http://server/ctx.tar.gz to the Docker daemon, which downloads and extracts the referenced tarball. The -f ctx/Dockerfile parameter specifies a path inside ctx.tar.gz to the Dockerfile that is used to build the image. Any ADD commands in that Dockerfile that refers to local paths must be relative to the root of the contents inside ctx.tar.gz. In the example above, the tarball contains a directory ctx/, so the ADD ctx/container.cfg / operation works as expected.

Build with -

$ docker build - < Dockerfile

This will read a Dockerfile from STDIN without context. Due to the lack of a context, no contents of any local directory will be sent to the Docker daemon. Since there is no context, a Dockerfile ADD only works if it refers to a remote URL.

$ docker build - < context.tar.gz

This will build an image for a compressed context read from STDIN. Supported formats are: bzip2, gzip and xz.

Use a .dockerignore file

$ docker build .

Uploading context 18.829 MB
Uploading context
Step 1/2 : FROM busybox
 ---> 769b9341d937
Step 2/2 : CMD echo Hello world
 ---> Using cache
 ---> 99cc1ad10469
Successfully built 99cc1ad10469
$ echo ".git" > .dockerignore
$ docker build .
Uploading context  6.76 MB
Uploading context
Step 1/2 : FROM busybox
 ---> 769b9341d937
Step 2/2 : CMD echo Hello world
 ---> Using cache
 ---> 99cc1ad10469
Successfully built 99cc1ad10469

This 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. The builder reference contains detailed information on creating a .dockerignore file

Tag an image (-t)

$ docker build -t vieux/apache:2.0 .

This will build like the previous example, but it will then tag the resulting image. The repository name will be vieux/apache and the tag will be 2.0. Read more about valid tags.

You can apply multiple tags to an image. For example, you can apply the latest tag to a newly built image and add another tag that references a specific version. For example, to tag an image both as whenry/fedora-jboss:latest and whenry/fedora-jboss:v2.1, use the following:

$ docker build -t whenry/fedora-jboss:latest -t whenry/fedora-jboss:v2.1 .

Specify a Dockerfile (-f)

$ docker build -f Dockerfile.debug .

This will use a file called Dockerfile.debug for the build instructions instead of Dockerfile.

$ curl example.com/remote/Dockerfile | docker build -f - .

The above command will use the current directory as the build context and read a Dockerfile from stdin.

$ docker build -f dockerfiles/Dockerfile.debug -t myapp_debug .
$ docker build -f dockerfiles/Dockerfile.prod  -t myapp_prod .

The above commands will build the current build context (as specified by the .) twice, once using a debug version of a Dockerfile and once using a production version.

$ cd /home/me/myapp/some/dir/really/deep
$ docker build -f /home/me/myapp/dockerfiles/debug /home/me/myapp
$ docker build -f ../../../../dockerfiles/debug /home/me/myapp

These two docker build commands do the exact same thing. They both use the contents of the debug file instead of looking for a Dockerfile and will use /home/me/myapp as the root of the build context. Note that debug is in the directory structure of the build context, regardless of how you refer to it on the command line.

Note: docker build will return a no such file or directory error if the file or directory does not exist in the uploaded context. This may happen if there is no context, or if you specify a file that is elsewhere on the Host system. The context is limited to the current directory (and its children) for security reasons, and to ensure repeatable builds on remote Docker hosts. This is also the reason why ADD ../file will not work.

Use a custom parent cgroup (–cgroup-parent)

When docker build is run with the --cgroup-parent option the containers used in the build will be run with the corresponding docker run flag.

Set ulimits in container (–ulimit)

Using the --ulimit option with docker build will cause each build step’s container to be started using those --ulimit flag values.

Set build-time variables (–build-arg)

You can use ENV instructions in a Dockerfile to define variable values. These values persist in the built image. However, often persistence is not what you want. Users want to specify variables differently depending on which host they build an image on.

A good example is http_proxy or source versions for pulling intermediate files. The ARG instruction lets Dockerfile authors define values that users can set at build-time using the --build-arg flag:

$ docker build --build-arg HTTP_PROXY=http://10.20.30.2:1234 .

This flag allows you to pass the build-time variables that are accessed like regular environment variables in the RUN instruction of the Dockerfile. Also, these values don’t persist in the intermediate or final images like ENV values do.

Using this flag will not alter the output you see when the ARG lines from the Dockerfile are echoed during the build process.

For detailed information on using ARG and ENV instructions, see the Dockerfile reference.

Optional security options (–security-opt)

This flag is only supported on a daemon running on Windows, and only supports the credentialspec option. The credentialspec must be in the format file://spec.txt or registry://keyname.

Specify isolation technology for container (–isolation)

This option is useful in situations where you are running Docker containers on Windows. The --isolation=<value> option sets a container’s isolation technology. On Linux, the only supported is the default option which uses Linux namespaces. On Microsoft Windows, you can specify these values:

Specifying the --isolation flag without a value is the same as setting --isolation="default".

Add entries to container hosts file (–add-host)

You can add other hosts into a container’s /etc/hosts file by using one or more --add-host flags. This example adds a static address for a host named docker:

$ docker build --add-host=docker:10.180.0.1 .

Specifying target build stage (–target)

When building a Dockerfile with multiple build stages, --target can be used to specify an intermediate build stage by name as a final stage for the resulting image. Commands after the target stage will be skipped.

FROM debian AS build-env
...

FROM alpine AS production-env
...

$ docker build -t mybuildimage --target build-env .

Squash an image’s layers (–squash) (experimental)

Overview

Once the image is built, squash the new layers into a new image with a single new layer. Squashing does not destroy any existing image, rather it creates a new image with the content of the squashed layers. This effectively makes it look like all Dockerfile commands were created with a single layer. The build cache is preserved with this method.

The --squash option is an experimental feature, and should not be considered stable.

Squashing layers can be beneficial if your Dockerfile produces multiple layers modifying the same files, for example, file that are created in one step, and removed in another step. For other use-cases, squashing images may actually have a negative impact on performance; when pulling an image consisting of multiple layers, layers can be pulled in parallel, and allows sharing layers between images (saving space).

For most use cases, multi-stage are a better alternative, as they give more fine-grained control over your build, and can take advantage of future optimizations in the builder. Refer to the use multi-stage builds section in the userguide for more information.

Known limitations

The --squash option has a number of known limitations:

• When squashing layers, the resulting image cannot take advantage of layer sharing with other images, and may use significantly more space. Sharing the base image is still supported.
• When using this option you may see significantly more space used due to storing two copies of the image, one for the build cache with all the cache layers in tact, and one for the squashed version.
• While squashing layers may produce smaller images, it may have a negative impact on performance, as a single layer takes longer to extract, and downloading a single layer cannot be parallelized.
• When attempting to squash an image that does not make changes to the filesystem (for example, the Dockerfile only contains ENV instructions), the squash step will fail (see issue #33823

Prerequisites

The example on this page is using experimental mode in Docker 1.13.

Experimental mode can be enabled by using the --experimental flag when starting the Docker daemon or setting experimental: true in the daemon.json configuration file.

By default, experimental mode is disabled. To see the current configuration, use the docker version command.

Server:
 Version:      1.13.1
 API version:  1.26 (minimum version 1.12)
 Go version:   go1.7.5
 Git commit:   092cba3
 Built:        Wed Feb  8 06:35:24 2017
 OS/Arch:      linux/amd64
 Experimental: false

 [...]

To enable experimental mode, users need to restart the docker daemon with the experimental flag enabled.

Enable Docker experimental

Experimental features are now included in the standard Docker binaries as of version 1.13.0. For enabling experimental features, you need to start the Docker daemon with --experimental flag. You can also enable the daemon flag via /etc/docker/daemon.json. e.g.

{
    "experimental": true
}

Then make sure the experimental flag is enabled:

$ docker version -f '{{.Server.Experimental}}'
true

Build an image with --squash argument

The following is an example of docker build with --squash argument

FROM busybox
RUN echo hello > /hello
RUN echo world >> /hello
RUN touch remove_me /remove_me
ENV HELLO world
RUN rm /remove_me

An image named test is built with --squash argument.

$ docker build --squash -t test .

[...]

If everything is right, the history will look like this:

$ docker history test 

IMAGE               CREATED             CREATED BY                                      SIZE                COMMENT
4e10cb5b4cac        3 seconds ago                                                       12 B                merge sha256:88a7b0112a41826885df0e7072698006ee8f621c6ab99fca7fe9151d7b599702 to sha256:47bcc53f74dc94b1920f0b34f6036096526296767650f223433fe65c35f149eb
<missing>           5 minutes ago       /bin/sh -c rm /remove_me                        0 B
<missing>           5 minutes ago       /bin/sh -c #(nop) ENV HELLO=world               0 B
<missing>           5 minutes ago       /bin/sh -c touch remove_me /remove_me           0 B
<missing>           5 minutes ago       /bin/sh -c echo world >> /hello                 0 B
<missing>           6 minutes ago       /bin/sh -c echo hello > /hello                  0 B
<missing>           7 weeks ago         /bin/sh -c #(nop) CMD ["sh"]                    0 B
<missing>           7 weeks ago         /bin/sh -c #(nop) ADD file:47ca6e777c36a4cfff   1.113 MB

We could find that all layer’s name is <missing>, and there is a new layer with COMMENT merge.

Test the image, check for /remove_me being gone, make sure hello\nworld is in /hello, make sure the HELLO envvar’s value is world.