Build context

The docker build and docker buildx build commands build Docker images from a Dockerfile and a context.

What is a build context?

The build context is the set of files that your build can access. The positional argument that you pass to the build command specifies the context that you want to use for the build:

$ docker build [OPTIONS] PATH | URL | -
                         ^^^^^^^^^^^^^^

You can pass any of the following inputs as the context for a build:

• The relative or absolute path to a local directory
• A remote URL of a Git repository, tarball, or plain-text file
• A plain-text file or tarball piped to the docker build command through standard input

Filesystem contexts

When your build context is a local directory, a remote Git repository, or a tar file, then that becomes the set of files that the builder can access during the build. Build instructions such as COPY and ADD can refer to any of the files and directories in the context.

A filesystem build context is processed recursively:

• When you specify a local directory or a tarball, all subdirectories are included
• When you specify a remote Git repository, the repository and all submodules are included

For more information about the different types of filesystem contexts that you can use with your builds, see:

• Local files
• Git repositories
• Remote tarballs

Text file contexts

When your build context is a plain-text file, the builder interprets the file as a Dockerfile. With this approach, the build doesn't use a filesystem context.

For more information, see empty build context.

Local context

To use a local build context, you can specify a relative or absolute filepath to the docker build command. The following example shows a build command that uses the current directory (.) as a build context:

$ docker build .
...
#16 [internal] load build context
#16 sha256:23ca2f94460dcbaf5b3c3edbaaa933281a4e0ea3d92fe295193e4df44dc68f85
#16 transferring context: 13.16MB 2.2s done
...

This makes files and directories in the current working directory available to the builder. The builder loads the files it needs from the build context when needed.

You can also use local tarballs as build context, by piping the tarball contents to the docker build command. See Tarballs.

Local directories

Consider the following directory structure:

.
├── index.ts
├── src/
├── Dockerfile
├── package.json
└── package-lock.json

Dockerfile instructions can reference and include these files in the build if you pass this directory as a context.

# syntax=docker/dockerfile:1
FROM node:latest
WORKDIR /src
COPY package.json package-lock.json .
RUN npm ci
COPY index.ts src .

$ docker build .

Local context with Dockerfile from stdin

Use the following syntax to build an image using files on your local filesystem, while using a Dockerfile from stdin.

$ docker build -f- <PATH>

The syntax uses the -f (or --file) option to specify the Dockerfile to use, and it uses a hyphen (-) as filename to instruct Docker to read the Dockerfile from stdin.

The following example uses the current directory (.) as the build context, and builds an image using a Dockerfile passed through stdin using a here-document.

# create a directory to work in
mkdir example
cd example

# create an example file
touch somefile.txt

# build an image using the current directory as context
# and a Dockerfile passed through stdin
docker build -t myimage:latest -f- . <<EOF
FROM busybox
COPY somefile.txt ./
RUN cat /somefile.txt
EOF

Local tarballs

When you pipe a tarball to the build command, the build uses the contents of the tarball as a filesystem context.

For example, given the following project directory:

.
├── Dockerfile
├── Makefile
├── README.md
├── main.c
├── scripts
├── src
└── test.Dockerfile

You can create a tarball of the directory and pipe it to the build for use as a context:

$ tar czf foo.tar.gz *
$ docker build - < foo.tar.gz

The build resolves the Dockerfile from the tarball context. You can use the --file flag to specify the name and location of the Dockerfile relative to the root of the tarball. The following command builds using test.Dockerfile in the tarball:

$ docker build --file test.Dockerfile - < foo.tar.gz

Remote context

You can specify the address of a remote Git repository, tarball, or plain-text file as your build context.

• For Git repositories, the builder automatically clones the repository. See Git repositories.
• For tarballs, the builder downloads and extracts the contents of the tarball. See Tarballs.

If the remote tarball is a text file, the builder receives no filesystem context, and instead assumes that the remote file is a Dockerfile. See Empty build context.

Git repositories

When you pass a URL pointing to the location of a Git repository as an argument to docker build, the builder uses the repository as the build context.

The builder performs a shallow clone of the repository, downloading only the HEAD commit, not the entire history.

The builder recursively clones the repository and any submodules it contains.

$ docker build https://github.com/user/myrepo.git

By default, the builder clones the latest commit on the default branch of the repository that you specify.

URL fragments

You can append URL fragments to the Git repository address to make the builder clone a specific branch, tag, and subdirectory of a repository.

The format of the URL fragment is #ref:dir, where:

• ref is the name of the branch, tag, or commit hash
• dir is a subdirectory inside the repository

For example, the following command uses the container branch, and the docker subdirectory in that branch, as the build context:

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

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

When you use a commit hash as the ref in the URL fragment, use the full, 40-character string SHA-1 hash of the commit. A short hash, for example a hash truncated to 7 characters, is not supported.

# ✅ The following works:
docker build github.com/docker/buildx#d4f088e689b41353d74f1a0bfcd6d7c0b213aed2
# ❌ The following doesn't work because the commit hash is truncated:
docker build github.com/docker/buildx#d4f088e

Keep .git directory

By default, BuildKit doesn't keep the .git directory when using Git contexts. You can configure BuildKit to keep the directory by setting the BUILDKIT_CONTEXT_KEEP_GIT_DIR build argument. This can be useful to if you want to retrieve Git information during your build:

# syntax=docker/dockerfile:1
FROM alpine
WORKDIR /src
RUN --mount=target=. \
  make REVISION=$(git rev-parse HEAD) build

$ docker build \
  --build-arg BUILDKIT_CONTEXT_KEEP_GIT_DIR=1
  https://github.com/user/myrepo.git#main

Private repositories

When you specify a Git context that's also a private repository, the builder needs you to provide the necessary authentication credentials. You can use either SSH or token-based authentication.

Buildx automatically detects and uses SSH credentials if the Git context you specify is an SSH or Git address. By default, this uses $SSH_AUTH_SOCK. You can configure the SSH credentials to use with the --ssh flag.

$ docker buildx build --ssh default git@github.com:user/private.git

If you want to use token-based authentication instead, you can pass the token using the --secret flag.

$ GIT_AUTH_TOKEN=<token> docker buildx build \
  --secret id=GIT_AUTH_TOKEN \
  https://github.com/user/private.git

メモ

Don't use --build-arg for secrets.

Remote context with Dockerfile from stdin

Use the following syntax to build an image using files on your local filesystem, while using a Dockerfile from stdin.

$ docker build -f- <URL>

The syntax uses the -f (or --file) option to specify the Dockerfile to use, and it uses a hyphen (-) as filename to instruct Docker to read the Dockerfile from stdin.

This can be useful in situations where you want to build an image from a repository that doesn't contain a Dockerfile. Or if you want to build with a custom Dockerfile, without maintaining your own fork of the repository.

The following example builds an image using a Dockerfile from stdin, and adds the hello.c file from the hello-world repository on GitHub.

docker build -t myimage:latest -f- https://github.com/docker-library/hello-world.git <<EOF
FROM busybox
COPY hello.c ./
EOF

Remote tarballs

If you pass the URL to a remote tarball, the URL itself is sent to the builder.

$ docker build http://server/context.tar.gz
#1 [internal] load remote build context
#1 DONE 0.2s

#2 copy /context /
#2 DONE 0.1s
...

The download operation will be performed on the host where the BuildKit daemon is running. Note that if you're using a remote Docker context or a remote builder, that's not necessarily the same machine as where you issue the build command. BuildKit fetches the context.tar.gz and uses 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.

Empty context

When you use a text file as the build context, the builder interprets the file as a Dockerfile. Using a text file as context means that the build has no filesystem context.

You can build with an empty build context when your Dockerfile doesn't depend on any local files.

How to build without a context

You can pass the text file using a standard input stream, or by pointing at the URL of a remote text file.

$ docker build - < Dockerfile

Get-Content Dockerfile | docker build -

docker build -t myimage:latest - <<EOF
FROM busybox
RUN echo "hello world"
EOF

$ docker build https://raw.githubusercontent.com/dvdksn/clockbox/main/Dockerfile

When you build without a filesystem context, Dockerfile instructions such as COPY can't refer to local files:

$ ls
main.c
$ docker build -<<< $'FROM scratch\nCOPY main.c .'
[+] Building 0.0s (4/4) FINISHED
 => [internal] load build definition from Dockerfile       0.0s
 => => transferring dockerfile: 64B                        0.0s
 => [internal] load .dockerignore                          0.0s
 => => transferring context: 2B                            0.0s
 => [internal] load build context                          0.0s
 => => transferring context: 2B                            0.0s
 => ERROR [1/1] COPY main.c .                              0.0s
------
 > [1/1] COPY main.c .:
------
Dockerfile:2
--------------------
   1 |     FROM scratch
   2 | >>> COPY main.c .
   3 |
--------------------
ERROR: failed to solve: failed to compute cache key: failed to calculate checksum of ref 7ab2bb61-0c28-432e-abf5-a4c3440bc6b6::4lgfpdf54n5uqxnv9v6ymg7ih: "/main.c": not found

.dockerignore files

You can use a .dockerignore file to exclude files or directories from the build context.

# .dockerignore
node_modules
bar

This helps avoid sending unwanted files and directories to the builder, improving build speed, especially when using a remote builder.

Filename and location

When you run a build command, the build client looks for a file named .dockerignore in the root directory of the context. If this file exists, the files and directories that match patterns in the files are removed from the build context before it's sent to the builder.

If you use multiple Dockerfiles, you can use different ignore-files for each Dockerfile. You do so using a special naming convention for the ignore-files. Place your ignore-file in the same directory as the Dockerfile, and prefix the ignore-file with the name of the Dockerfile, as shown in the following example.

.
├── index.ts
├── src/
├── docker
│   ├── build.Dockerfile
│   ├── build.Dockerfile.dockerignore
│   ├── lint.Dockerfile
│   ├── lint.Dockerfile.dockerignore
│   ├── test.Dockerfile
│   └── test.Dockerfile.dockerignore
├── package.json
└── package-lock.json

A Dockerfile-specific ignore-file takes precedence over the .dockerignore file at the root of the build context if both exist.

Syntax

The .dockerignore file is a newline-separated list of patterns similar to the file globs of Unix shells. Leading and trailing slashes in ignore patterns are disregarded. The following patterns all exclude a file or directory named bar in the subdirectory foo under the root of the build context:

• /foo/bar/
• /foo/bar
• foo/bar/
• foo/bar

If a line in .dockerignore file starts with # in column 1, then this line is considered as a comment and is ignored before interpreted by the CLI.

#/this/is/a/comment

If you're interested in learning the precise details of the .dockerignore pattern matching logic, check out the moby/patternmatcher repository on GitHub, which contains the source code.

Matching

The following code snippet shows an example .dockerignore file.

# comment
*/temp*
*/*/temp*
temp?

This file causes the following build behavior:

Matching is done using Go's filepath.Match function rules. A preprocessing step uses Go's filepath.Clean function to trim whitespace and remove . and ... Lines that are blank after preprocessing are ignored.

メモ

For historical reasons, the pattern . is ignored.

Beyond Go's filepath.Match rules, Docker also supports a special wildcard string ** that matches any number of directories (including zero). For example, **/*.go excludes all files that end with .go found anywhere in the build context.

You can use the .dockerignore file to exclude the Dockerfile and .dockerignore files. These files are still sent to the builder as they're needed for running the build. But you can't copy the files into the image using ADD, COPY, or bind mounts.

Negating matches

You can prepend lines with a ! (exclamation mark) to make exceptions to exclusions. The following is an example .dockerignore file that uses this mechanism:

*.md
!README.md

All markdown files right under the context directory except README.md are excluded from the context. Note that markdown files under subdirectories are still included.

The placement of ! exception rules influences the behavior: the last line of the .dockerignore that matches a particular file determines whether it's included or excluded. Consider the following example:

*.md
!README*.md
README-secret.md

No markdown files are included in the context except README files other than README-secret.md.

Now consider this example:

*.md
README-secret.md
!README*.md

All of the README files are included. The middle line has no effect because !README*.md matches README-secret.md and comes last.