buildx/docs/bake-reference.md

Bake file reference

The Bake file is a file for defining workflows that you run using docker buildx bake.

File format

You can define your Bake file in the following file formats:

• HashiCorp Configuration Language (HCL)
• JSON
• YAML (Compose file)

By default, Bake uses the following lookup order to find the configuration file:

1. docker-bake.override.hcl
2. docker-bake.hcl
3. docker-bake.override.json
4. docker-bake.json
5. docker-compose.yaml
6. docker-compose.yml

Bake searches for the file in the current working directory. You can specify the file location explicitly using the --file flag:

$ docker buildx bake --file=../docker/bake.hcl --print

Syntax

The Bake file supports the following property types:

• target: build targets
• group: collections of build targets
• variable: build arguments and variables
• function: custom Bake functions

You define properties as hierarchical blocks in the Bake file. You can assign one or more attributes to a property.

The following snippet shows a JSON representation of a simple Bake file. This Bake file defines three properties: a variable, a group, and a target.

{
  "variable": {
    "TAG": {
      "default": "latest"
    }
  },
  "group": {
    "default": {
      "targets": ["webapp"]
    }
  },
  "target": {
    "webapp": {
      "dockerfile": "Dockerfile",
      "tags": ["docker.io/username/webapp:${TAG}"]
    }
  }
}

In the JSON representation of a Bake file, properties are objects, and attributes are values assigned to those objects.

The following example shows the same Bake file in the HCL format:

variable "TAG" {
  default = "latest"
}

group "default" {
  targets = ["webapp"]
}

target "webapp" {
  dockerfile = "Dockerfile"
  tags = ["docker.io/username/webapp:${TAG}"]
}

HCL is the preferred format for Bake files. Aside from syntactic differences, HCL lets you use features that the JSON and YAML formats don't support.

The examples in this document use the HCL format.

Target

A target reflects a single docker build invocation. Consider the following build command:

$ docker build \
  --file=Dockerfile.webapp \
  --tag=docker.io/username/webapp:latest \
  https://github.com/username/webapp

You can express this command in a Bake file as follows:

target "webapp" {
  dockerfile = "Dockerfile.webapp"
  tags = ["docker.io/username/webapp:latest"]
  context = "https://github.com/username/webapp"
}

The following table shows the complete list of attributes that you can assign to a target:

Name	Type	Description
args	Map	Build arguments
attest	List	Build attestations
cache-from	List	External cache sources
cache-to	List	External cache destinations
context	String	Set of files located in the specified path or URL
contexts	Map	Additional build contexts
dockerfile-inline	String	Inline Dockerfile string
dockerfile	String	Dockerfile location
inherits	List	Inherit attributes from other targets
labels	Map	Metadata for images
matrix	Map	Define a set of variables that forks a target into multiple targets.
name	String	Override the target name when using a matrix.
no-cache-filter	List	Disable build cache for specific stages
no-cache	Boolean	Disable build cache completely
output	List	Output destinations
platforms	List	Target platforms
pull	Boolean	Always pull images
secret	List	Secrets to expose to the build
ssh	List	SSH agent sockets or keys to expose to the build
tags	List	Image names and tags
target	String	Target build stage

target.args

Use the args attribute to define build arguments for the target. This has the same effect as passing a --build-arg flag to the build command.

target "default" {
  args = {
    VERSION = "0.0.0+unknown"
  }
}

You can set args attributes to use null values. Doing so forces the target to use the ARG value specified in the Dockerfile.

variable "GO_VERSION" {
  default = "1.20.3"
}

target "webapp" {
  dockerfile = "webapp.Dockerfile"
  tags = ["docker.io/username/webapp"]
}

target "db" {
  args = {
    GO_VERSION = null
  }
  dockerfile = "db.Dockerfile"
  tags = ["docker.io/username/db"]
}

target.attest

The attest attribute lets you apply build attestations to the target. This attribute accepts the long-form CSV version of attestation parameters.

target "default" {
  attest = [
    "type=provenance,mode=min",
    "type=sbom"
  ]
}

target.cache-from

Build cache sources. The builder imports cache from the locations you specify. It uses the Buildx cache storage backends, and it works the same way as the --cache-from flag. This takes a list value, so you can specify multiple cache sources.

target "app" {
  cache-from = [
    "type=s3,region=eu-west-1,bucket=mybucket",
    "user/repo:cache",
  ]
}

target.cache-to

Build cache export destinations. The builder exports its build cache to the locations you specify. It uses the Buildx cache storage backends, and it works the same way as the --cache-to flag. This takes a list value, so you can specify multiple cache export targets.

target "app" {
  cache-to = [
    "type=s3,region=eu-west-1,bucket=mybucket",
    "type=inline"
  ]
}

target.context

Specifies the location of the build context to use for this target. Accepts a URL or a directory path. This is the same as the build context positional argument that you pass to the build command.

target "app" {
  context = "./src/www"
}

This resolves to the current working directory (".") by default.

$ docker buildx bake --print -f - <<< 'target "default" {}'
[+] Building 0.0s (0/0)
{
  "target": {
    "default": {
      "context": ".",
      "dockerfile": "Dockerfile"
    }
  }
}

target.contexts

Additional build contexts. This is the same as the --build-context flag. This attribute takes a map, where keys result in named contexts that you can reference in your builds.

You can specify different types of contexts, such local directories, Git URLs, and even other Bake targets. Bake automatically determines the type of a context based on the pattern of the context value.

Context type	Example
Container image	docker-image://alpine@sha256:0123456789
Git URL	https://github.com/user/proj.git
HTTP URL	https://example.com/files
Local directory	../path/to/src
Bake target	target:base

Pin an image version

# docker-bake.hcl
target "app" {
    contexts = {
        alpine = "docker-image://alpine:3.13"
    }
}

# Dockerfile
FROM alpine
RUN echo "Hello world"

Use a local directory

# docker-bake.hcl
target "app" {
    contexts = {
        src = "../path/to/source"
    }
}

# Dockerfile
FROM scratch AS src
FROM golang
COPY --from=src . .

Use another target as base

Note

You should prefer to use regular multi-stage builds over this option. You can Use this feature when you have multiple Dockerfiles that can't be easily merged into one.

# docker-bake.hcl
target "base" {
    dockerfile = "baseapp.Dockerfile"
}
target "app" {
    contexts = {
        baseapp = "target:base"
    }
}

# Dockerfile
FROM baseapp
RUN echo "Hello world"

target.dockerfile-inline

Uses the string value as an inline Dockerfile for the build target.

target "default" {
  dockerfile-inline = "FROM alpine\nENTRYPOINT [\"echo\", \"hello\"]"
}

The dockerfile-inline takes precedence over the dockerfile attribute. If you specify both, Bake uses the inline version.

target.dockerfile

Name of the Dockerfile to use for the build. This is the same as the --file flag for the docker build command.

target "default" {
  dockerfile = "./src/www/Dockerfile"
}

Resolves to "Dockerfile" by default.

$ docker buildx bake --print -f - <<< 'target "default" {}'
[+] Building 0.0s (0/0)
{
  "target": {
    "default": {
      "context": ".",
      "dockerfile": "Dockerfile"
    }
  }
}

target.inherits

A target can inherit attributes from other targets. Use inherits to reference from one target to another.

In the following example, the app-dev target specifies an image name and tag. The app-release target uses inherits to reuse the tag name.

variable "TAG" {
  default = "latest"
}

target "app-dev" {
  tags = ["docker.io/username/myapp:${TAG}"]
}

target "app-release" {
  inherits = ["app-dev"]
  platforms = ["linux/amd64", "linux/arm64"]
}

The inherits attribute is a list, meaning you can reuse attributes from multiple other targets. In the following example, the app-release target reuses attributes from both the app-dev and _release targets.

target "app-dev" {
  args = {
    GO_VERSION = "1.20"
    BUILDX_EXPERIMENTAL = 1
  }
  tags = ["docker.io/username/myapp"]
  dockerfile = "app.Dockerfile"
  labels = {
    "org.opencontainers.image.source" = "https://github.com/username/myapp"
  }
}

target "_release" {
  args = {
    BUILDKIT_CONTEXT_KEEP_GIT_DIR = 1
    BUILDX_EXPERIMENTAL = 0
  }
}

target "app-release" {
  inherits = ["app-dev", "_release"]
  platforms = ["linux/amd64", "linux/arm64"]
}

When inheriting attributes from multiple targets and there's a conflict, the target that appears last in the inherits list takes precedence. The previous example defines the BUILDX_EXPERIMENTAL argument twice for the app-release target. It resolves to 0 because the _release target appears last in the inheritance chain:

$ docker buildx bake --print app-release
[+] Building 0.0s (0/0)
{
  "group": {
    "default": {
      "targets": [
        "app-release"
      ]
    }
  },
  "target": {
    "app-release": {
      "context": ".",
      "dockerfile": "app.Dockerfile",
      "args": {
        "BUILDKIT_CONTEXT_KEEP_GIT_DIR": "1",
        "BUILDX_EXPERIMENTAL": "0",
        "GO_VERSION": "1.20"
      },
      "labels": {
        "org.opencontainers.image.source": "https://github.com/username/myapp"
      },
      "tags": [
        "docker.io/username/myapp"
      ],
      "platforms": [
        "linux/amd64",
        "linux/arm64"
      ]
    }
  }
}

target.labels

Assigns image labels to the build. This is the same as the --label flag for docker build.

target "default" {
  labels = {
    "org.opencontainers.image.source" = "https://github.com/username/myapp"
    "com.docker.image.source.entrypoint" = "Dockerfile"
  }
}

It's possible to use a null value for labels. If you do, the builder uses the label value specified in the Dockerfile.

target.matrix

A matrix strategy lets you fork a single target into multiple different variants, based on parameters that you specify. This works in a similar way to [Matrix strategies for GitHub Actions]. You can use this to reduce duplication in your bake definition.

The matrix attribute is a map of parameter names to lists of values. Bake builds each possible combination of values as a separate target.

Each generated target must have a unique name. To specify how target names should resolve, use the name attribute.

The following example resolves the app target to app-foo and app-bar. It also uses the matrix value to define the target build stage.

target "app" {
  name = "app-${tgt}"
  matrix = {
    tgt = ["foo", "bar"]
  }
  target = tgt
}

$ docker buildx bake --print app
[+] Building 0.0s (0/0)
{
  "group": {
    "app": {
      "targets": [
        "app-foo",
        "app-bar"
      ]
    },
    "default": {
      "targets": [
        "app"
      ]
    }
  },
  "target": {
    "app-bar": {
      "context": ".",
      "dockerfile": "Dockerfile",
      "target": "bar"
    },
    "app-foo": {
      "context": ".",
      "dockerfile": "Dockerfile",
      "target": "foo"
    }
  }
}

Multiple axes

You can specify multiple keys in your matrix to fork a target on multiple axes. When using multiple matrix keys, Bake builds every possible variant.

The following example builds four targets:

• app-foo-1-0
• app-foo-2-0
• app-bar-1-0
• app-bar-2-0

target "app" {
  name = "app-${tgt}-${replace(version, ".", "-")}"
  matrix = {
    tgt = ["foo", "bar"]
    version = ["1.0", "2.0"]
  }
  target = tgt
  args = {
    VERSION = version
  }
}

Multiple values per matrix target

If you want to differentiate the matrix on more than just a single value, you can use maps as matrix values. Bake creates a target for each map, and you can access the nested values using dot notation.

The following example builds two targets:

• app-foo-1-0
• app-bar-2-0

target "app" {
  name = "app-${item.tgt}-${replace(item.version, ".", "-")}"
  matrix = {
    item = [
      {
        tgt = "foo"
        version = "1.0"
      },
      {
        tgt = "bar"
        version = "2.0"
      }
    ]
  }
  target = item.tgt
  args = {
    VERSION = item.version
  }
}

target.name

Specify name resolution for targets that use a matrix strategy. The following example resolves the app target to app-foo and app-bar.

target "app" {
  name = "app-${tgt}"
  matrix = {
    tgt = ["foo", "bar"]
  }
  target = tgt
}

target.no-cache-filter

Don't use build cache for the specified stages. This is the same as the --no-cache-filter flag for docker build. The following example avoids build cache for the foo build stage.

target "default" {
  no-cache-filter = ["foo"]
}

target.no-cache

Don't use cache when building the image. This is the same as the --no-cache flag for docker build.

target "default" {
  no-cache = 1
}

target.output

Configuration for exporting the build output. This is the same as the --output flag. The following example configures the target to use a cache-only output,

target "default" {
  output = ["type=cacheonly"]
}

target.platforms

Set target platforms for the build target. This is the same as the --platform flag. The following example creates a multi-platform build for three architectures.

target "default" {
  platforms = ["linux/amd64", "linux/arm64", "linux/arm/v7"]
}

target.pull

Configures whether the builder should attempt to pull images when building the target. This is the same as the --pull flag for docker build. The following example forces the builder to always pull all images referenced in the build target.

target "default" {
  pull = "always"
}

target.secret

Defines secrets to expose to the build target. This is the same as the --secret flag.

variable "HOME" {
  default = null
}

target "default" {
  secret = [
    "type=env,id=KUBECONFIG",
    "type=file,id=aws,src=${HOME}/.aws/credentials"
  ]
}

This lets you mount the secret in your Dockerfile.

RUN --mount=type=secret,id=aws,target=/root/.aws/credentials \
    aws cloudfront create-invalidation ...
RUN --mount=type=secret,id=KUBECONFIG \
    KUBECONFIG=$(cat /run/secrets/KUBECONFIG) helm upgrade --install

target.ssh

Defines SSH agent sockets or keys to expose to the build. This is the same as the --ssh flag. This can be useful if you need to access private repositories during a build.

target "default" {
  ssh = ["default"]
}

FROM alpine
RUN --mount=type=ssh \
    apk add git openssh-client \
    && install -m 0700 -d ~/.ssh \
    && ssh-keyscan github.com >> ~/.ssh/known_hosts \
    && git clone git@github.com:user/my-private-repo.git

target.tags

Image names and tags to use for the build target. This is the same as the --tag flag.

target "default" {
  tags = [
    "org/repo:latest",
    "myregistry.azurecr.io/team/image:v1"
  ]
}

target.target

Set the target build stage to build. This is the same as the --target flag.

target "default" {
  target = "binaries"
}

Group

Groups allow you to invoke multiple builds (targets) at once.

group "default" {
  targets = ["db", "webapp-dev"]
}

target "webapp-dev" {
  dockerfile = "Dockerfile.webapp"
  tags = ["docker.io/username/webapp:latest"]
}

target "db" {
  dockerfile = "Dockerfile.db"
  tags = ["docker.io/username/db"]
}

Groups take precedence over targets, if both exist with the same name. The following bake file builds the default group. Bake ignores the default target.

target "default" {
  dockerfile-inline = "FROM ubuntu"
}

group "default" {
  targets = ["alpine", "debian"]
}
target "alpine" {
  dockerfile-inline = "FROM alpine"
}
target "debian" {
  dockerfile-inline = "FROM debian"
}

Variable

The HCL file format supports variable block definitions. You can use variables as build arguments in your Dockerfile, or interpolate them in attribute values in your Bake file.

variable "TAG" {
  default = "latest"
}

target "webapp-dev" {
  dockerfile = "Dockerfile.webapp"
  tags = ["docker.io/username/webapp:${TAG}"]
}

You can assign a default value for a variable in the Bake file, or assign a null value to it. If you assign a null value, Buildx uses the default value from the Dockerfile instead.

You can override variable defaults set in the Bake file using environment variables. The following example sets the TAG variable to dev, overriding the default latest value shown in the previous example.

$ TAG=dev docker buildx bake webapp-dev

Built-in variables

The following variables are built-ins that you can use with Bake without having to define them.

Variable	Description
BAKE_CMD_CONTEXT	Holds the main context when building using a remote Bake file.
BAKE_LOCAL_PLATFORM	Returns the current platform’s default platform specification (e.g. linux/amd64).

Use environment variable as default

You can set a Bake variable to use the value of an environment variable as a default value:

variable "HOME" {
  default = "$HOME"
}

Interpolate variables into attributes

To interpolate a variable into an attribute string value, you must use curly brackets. The following doesn't work:

variable "HOME" {
  default = "$HOME"
}

target "default" {
  ssh = ["default=$HOME/.ssh/id_rsa"]
}

Wrap the variable in curly brackets where you want to insert it:

  variable "HOME" {
    default = "$HOME"
  }

  target "default" {
-   ssh = ["default=$HOME/.ssh/id_rsa"]
+   ssh = ["default=${HOME}/.ssh/id_rsa"]
  }

Before you can interpolate a variable into an attribute, first you must declare it in the bake file, as demonstrated in the following example.

$ cat docker-bake.hcl
target "default" {
  dockerfile-inline = "FROM ${BASE_IMAGE}"
}
$ docker buildx bake
[+] Building 0.0s (0/0)
docker-bake.hcl:2
--------------------
   1 |     target "default" {
   2 | >>>   dockerfile-inline = "FROM ${BASE_IMAGE}"
   3 |     }
   4 |
--------------------
ERROR: docker-bake.hcl:2,31-41: Unknown variable; There is no variable named "BASE_IMAGE"., and 1 other diagnostic(s)
$ cat >> docker-bake.hcl

variable "BASE_IMAGE" {
  default = "alpine"
}

$ docker buildx bake
[+] Building 0.6s (5/5) FINISHED

Function

A set of general-purpose functions provided by go-cty are available for use in HCL files:

# docker-bake.hcl
target "webapp-dev" {
  dockerfile = "Dockerfile.webapp"
  tags = ["docker.io/username/webapp:latest"]
  args = {
    buildno = "${add(123, 1)}"
  }
}

In addition, user defined functions are also supported:

# docker-bake.hcl
function "increment" {
  params = [number]
  result = number + 1
}

target "webapp-dev" {
  dockerfile = "Dockerfile.webapp"
  tags = ["docker.io/username/webapp:latest"]
  args = {
    buildno = "${increment(123)}"
  }
}

Note

See User defined HCL functions page for more details.