docker build

Description

Build an image from a Dockerfile

Usage

docker build [OPTIONS] PATH | URL | -

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:

Note

You cannot specify the build-context directory (myfolder in the examples above) when using BuildKit as builder (DOCKER_BUILDKIT=1). Support for this feature is tracked in buildkit#1684.

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.

For example uses of this command, refer to the examples section below.

Options

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.

When using the BuildKit backend, docker build searches for a .dockerignore file relative to the Dockerfile name. For example, running docker build -f myapp.Dockerfile . will first look for an ignore file named myapp.Dockerfile.dockerignore. If such a file is not found, the .dockerignore file is used if present. Using a Dockerfile based .dockerignore is useful if a project contains multiple Dockerfiles that expect to ignore different sets of files.

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 returns 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 does 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 --build-arg FTP_PROXY=http://40.50.60.5:4567 .

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. You must add --build-arg for each build argument.

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.

You may also use the --build-arg flag without a value, in which case the value from the local environment will be propagated into the Docker container being built:

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

This is similar to how docker run -e works. Refer to the docker run documentation for more information.

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 .

Custom build outputs

By default, a local container image is created from the build result. The --output (or -o) flag allows you to override this behavior, and a specify a custom exporter. For example, custom exporters allow you to export the build artifacts as files on the local filesystem instead of a Docker image, which can be useful for generating local binaries, code generation etc.

The value for --output is a CSV-formatted string defining the exporter type and options. Currently, local and tar exporters are supported. The local exporter writes the resulting build files to a directory on the client side. The tar exporter is similar but writes the files as a single tarball (.tar).

If no type is specified, the value defaults to the output directory of the local exporter. Use a hyphen (-) to write the output tarball to standard output (STDOUT).

The following example builds an image using the current directory (.) as build context, and exports the files to a directory named out in the current directory. If the directory does not exist, Docker creates the directory automatically:

$ docker build -o out .

The example above uses the short-hand syntax, omitting the type options, and thus uses the default (local) exporter. The example below shows the equivalent using the long-hand CSV syntax, specifying both type and dest (destination path):

$ docker build --output type=local,dest=out .

Use the tar type to export the files as a .tar archive:

$ docker build --output type=tar,dest=out.tar .

The example below shows the equivalent when using the short-hand syntax. In this case, - is specified as destination, which automatically selects the tar type, and writes the output tarball to standard output, which is then redirected to the out.tar file:

docker build -o - . > out.tar

The --output option exports all files from the target stage. A common pattern for exporting only specific files is to do multi-stage builds and to copy the desired files to a new scratch stage with COPY --from.

The example Dockerfile below uses a separate stage to collect the build-artifacts for exporting:

FROM golang AS build-stage
RUN go get -u github.com/LK4D4/vndr

FROM scratch AS export-stage
COPY --from=build-stage /go/bin/vndr /

When building the Dockerfile with the -o option, only the files from the final stage are exported to the out directory, in this case, the vndr binary:

$ docker build -o out .

[+] Building 2.3s (7/7) FINISHED
 => [internal] load build definition from Dockerfile                                                                          0.1s
 => => transferring dockerfile: 176B                                                                                          0.0s
 => [internal] load .dockerignore                                                                                             0.0s
 => => transferring context: 2B                                                                                               0.0s
 => [internal] load metadata for docker.io/library/golang:latest                                                              1.6s
 => [build-stage 1/2] FROM docker.io/library/golang@sha256:2df96417dca0561bf1027742dcc5b446a18957cd28eba6aa79269f23f1846d3f   0.0s
 => => resolve docker.io/library/golang@sha256:2df96417dca0561bf1027742dcc5b446a18957cd28eba6aa79269f23f1846d3f               0.0s
 => CACHED [build-stage 2/2] RUN go get -u github.com/LK4D4/vndr                                                              0.0s
 => [export-stage 1/1] COPY --from=build-stage /go/bin/vndr /                                                                 0.2s
 => exporting to client                                                                                                       0.4s
 => => copying files 10.30MB                                                                                                  0.3s

$ ls ./out
vndr

Note

This feature requires the BuildKit backend. You can either enable BuildKit or use the buildx plugin which provides more output type options.

Specifying external cache sources

In addition to local build cache, the builder can reuse the cache generated from previous builds with the --cache-from flag pointing to an image in the registry.

To use an image as a cache source, cache metadata needs to be written into the image on creation. This can be done by setting --build-arg BUILDKIT_INLINE_CACHE=1 when building the image. After that, the built image can be used as a cache source for subsequent builds.

Upon importing the cache, the builder will only pull the JSON metadata from the registry and determine possible cache hits based on that information. If there is a cache hit, the matched layers are pulled into the local environment.

In addition to images, the cache can also be pulled from special cache manifests generated by buildx or the BuildKit CLI (buildctl). These manifests (when built with the type=registry and mode=max options) allow pulling layer data for intermediate stages in multi-stage builds.

The following example builds an image with inline-cache metadata and pushes it to a registry, then uses the image as a cache source on another machine:

$ docker build -t myname/myapp --build-arg BUILDKIT_INLINE_CACHE=1 .
$ docker push myname/myapp

After pushing the image, the image is used as cache source on another machine. BuildKit automatically pulls the image from the registry if needed.

# on another machine
$ docker build --cache-from myname/myapp .

Note

This feature requires the BuildKit backend. You can either enable BuildKit or use the buildx plugin. The previous builder has limited support for reusing cache from pre-pulled images.

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, files 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 builds 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 19.03.

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 of the docker daemon, use the docker version command and check the Experimental line in the Engine section:

Client: Docker Engine - Community
 Version:           19.03.8
 API version:       1.40
 Go version:        go1.12.17
 Git commit:        afacb8b
 Built:             Wed Mar 11 01:21:11 2020
 OS/Arch:           darwin/amd64
 Experimental:      false

Server: Docker Engine - Community
 Engine:
  Version:          19.03.8
  API version:      1.40 (minimum version 1.12)
  Go version:       go1.12.17
  Git commit:       afacb8b
  Built:            Wed Mar 11 01:29:16 2020
  OS/Arch:          linux/amd64
  Experimental:     true
 [...]

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

Enable Docker experimental

To enable experimental features, you need to start the Docker daemon with --experimental flag. You can also enable the daemon flag via /etc/docker/daemon.json, for example:

{
    "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 looks 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 a 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 environment variable’s value is world.

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