Inheritance in Bake

Targets can inherit attributes from other targets, using the inherits attribute. For example, imagine that you have a target that builds a Docker image for a development environment:

target "app-dev" {
  args = {
    GO_VERSION = "1.21"
  }
  tags = ["docker.io/username/myapp:dev"]
  labels = {
    "org.opencontainers.image.source" = "https://github.com/username/myapp"
    "org.opencontainers.image.author" = "moby.whale@example.com"
  }
}

You can create a new target that uses the same build configuration, but with slightly different attributes for a production build. In this example, the app-release target inherits the app-dev target, but overrides the tags attribute and adds a new platforms attribute:

target "app-release" {
  inherits = ["app-dev"]
  tags = ["docker.io/username/myapp:latest"]
  platforms = ["linux/amd64", "linux/arm64"]
}

Common reusable targets

One common inheritance pattern is to define a common target that contains shared attributes for all or many of the build targets in the project. For example, the following _common target defines a common set of build arguments:

target "_common" {
  args = {
    GO_VERSION = "1.21"
    BUILDKIT_CONTEXT_KEEP_GIT_DIR = 1
  }
}

You can then inherit the _common target in other targets to apply the shared attributes:

target "lint" {
  inherits = ["_common"]
  dockerfile = "./dockerfiles/lint.Dockerfile"
  output = ["type=cacheonly"]
}

target "docs" {
  inherits = ["_common"]
  dockerfile = "./dockerfiles/docs.Dockerfile"
  output = ["./docs/reference"]
}

target "test" {
  inherits = ["_common"]
  target = "test-output"
  output = ["./test"]
}

target "binaries" {
  inherits = ["_common"]
  target = "binaries"
  output = ["./build"]
  platforms = ["local"]
}

Overriding inherited attributes

When a target inherits another target, it can override any of the inherited attributes. For example, the following target overrides the args attribute from the inherited target:

target "app-dev" {
  inherits = ["_common"]
  args = {
    GO_VERSION = "1.17"
  }
  tags = ["docker.io/username/myapp:dev"]
}

The GO_VERSION argument in app-release is set to 1.17, overriding the GO_VERSION argument from the app-dev target.

For more information about overriding attributes, see the Overriding configurations page.

Inherit from multiple targets

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 _common targets.

target "_common" {
  args = {
    GO_VERSION = "1.21"
    BUILDKIT_CONTEXT_KEEP_GIT_DIR = 1
  }
}

target "app-dev" {
  inherits = ["_common"]
  args = {
    BUILDKIT_CONTEXT_KEEP_GIT_DIR = 0
  }
  tags = ["docker.io/username/myapp:dev"]
  labels = {
    "org.opencontainers.image.source" = "https://github.com/username/myapp"
    "org.opencontainers.image.author" = "moby.whale@example.com"
  }
}

target "app-release" {
  inherits = ["app-dev", "_common"]
  tags = ["docker.io/username/myapp:latest"]
  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 BUILDKIT_CONTEXT_KEEP_GIT_DIR in the _common target and overrides it in the app-dev target.

The app-release target inherits both app-dev target and the _common target. The BUILDKIT_CONTEXT_KEEP_GIT_DIR argument is set to 0 in the app-dev target and 1 in the _common target. The BUILDKIT_CONTEXT_KEEP_GIT_DIR argument in the app-release target is set to 1, not 0, because the _common target appears last in the inherits list.

Reusing single attributes from targets

If you only want to inherit a single attribute from a target, you can reference an attribute from another target using dot notation. For example, in the following Bake file, the bar target reuses the tags attribute from the foo target:

target "foo" {
  dockerfile = "foo.Dockerfile"
  tags       = ["myapp:latest"]
}
target "bar" {
  dockerfile = "bar.Dockerfile"
  tags       = target.foo.tags
}