What is a stack?

A stack is composed of two images that are intended to work together:

  1. The build image of a stack provides the base image from which the build environment is constructed. The build environment is the containerized environment in which the lifecycle (and thereby buildpacks) are executed.
  2. The run image of a stack provides the base image from which application images are built.

If you’re using the pack CLI, running pack stack suggest will display a list of recommended stacks that can be used when running pack builder create, along with each stack’s associated build and run images.

Using stacks

Stacks are used by builders and are configured through a builder’s configuration file:

  # ...

  # ...

  id = "com.example.stack"
  build-image = "example/build"
  run-image = "example/run"
  run-image-mirrors = ["", ""]

By providing the required [stack] section, a builder author can configure a stack’s ID, build image, and run image (including any mirrors).

Run image mirrors

Run image mirrors provide alternate locations for run images, for use during build (or rebase). When running build with a builder containing run image mirrors, pack will select a run image whose registry location matches that of the specified app image (if no registry host is specified in the image name, DockerHub is assumed). This is useful when publishing the resulting app image (via the --publish flag or via docker push), as the app’s base image (i.e. run image) will be located on the same registry as the app image itself, reducing the amount of data transfer required to push the app image.

In the following example, assuming a builder configured with the example TOML above, the selected run image will be

$ pack build

while naming the app without a registry specified, example/app, will cause example/run to be selected as the app’s run image.

$ pack build example/app

For local development, it’s often helpful to override the run image mirrors in a builder. For this, the pack config run-image-mirrors command can be used. This command does not modify the builder, and instead configures the user’s local machine.

To see what run images are configured for a builder, the inspect-builder command can be used. inspect-builder will output built-in and locally-configured run images for a given builder, among other useful information. The order of the run images in the output denotes the order in which they will be matched during build.

Creating custom stacks

To create a custom stack, simply create customized build and run images containing the following information:


Name Description Format Identifier for the stack String

Environment Variables

Name Description
CNB_STACK_ID Identifier for the stack
CNB_USER_ID UID of the user specified in the image
CNB_GROUP_ID GID of the user specified in the image

NOTE: The stack identifier implies compatibility with other stacks of that same identifier. For instance, a custom stack may use io.buildpacks.stacks.bionic as its identifier so long as it will work with buildpacks that declare compatibility with the io.buildpacks.stacks.bionic stack.


Mixins provide a way to document OS-level dependencies that a stack provides to buildpacks. Mixins can be provided at build-time (name prefixed with build:), run-time (name prefixed with run:), or both (name unprefixed).

Declaring provided mixins

When declaring provided mixins, both the build and run image of a stack must contain the following label:

Name Description Format
io.buildpacks.stack.mixins List of provided mixins JSON string array

The following rules apply for mixin declarations:

  • build:-prefixed mixins may not be declared on a run image
  • run:-prefixed mixins may not be declared on a build image
  • Unprefixed mixins must be declared on both stack images


Build image:

io.buildpacks.stack.mixins: ["build:git", "wget"]

Run image:

io.buildpacks.stack.mixins: ["run:imagemagick", "wget"]

Declaring required mixins

A buildpack must list any required mixins in the stacks section of its buildpack.toml file.

When validating whether the buildpack’s mixins are satisfied by a stack, the following rules apply:

  • build:-prefixed mixins must be provided by stack’s build image
  • run:-prefixed mixins must be provided by stack’s run image
  • Unprefixed mixins must be provided by both stack images


id = "io.buildpacks.stacks.bionic"
mixins = ["build:git", "run:imagemagick", "wget"]


For sample stacks, see our samples repo.

For technical details on stacks, see the platform specification for stacks.