Cache images are a way to preserve build-optimizing layers across different host machines. These images can improve performance when using pack in ephemeral environments such as CI/CD pipelines.

Using Cache Images (--cache-image)

The --cache-image parameter must be in the following format

--cache-image <remote-image-location>

The --cache-image flag must be specified in conjunction with the --publish flag.

Examples

For the following examples we will use:

• A local Docker v2 registry running on port 5000
• Our samples repo

NOTE: If we wish to publish to an external registry like Dockerhub we will first need to authenticate with docker to allow us to push images. We can do this via docker login

Next we trust the cnbs/sample-builder:jammy builder in order to allow access to docker credentials when publishing.

pack config trusted-builders add cnbs/sample-builder:jammy

To build the localhost:5000/buildpack-examples/cache-image-example application image and the localhost:5000/buildpack-examples/maven-cache-image:latest cache image we may run the following

pack build localhost:5000/buildpack-examples/cache-image-example \
    --builder cnbs/sample-builder:jammy \
    --buildpack samples/java-maven \
    --path samples/apps/java-maven \
    --cache-image localhost:5000/buildpack-examples/maven-cache-image:latest \
    --network host \
    --publish

NOTE: Please omit --network host if you are using a remote registry like Dockerhub or on Windows.

Now we may inspect both the application image, and the cache image by pulling them -

Let’s inspect the application image first -

docker pull localhost:5000/buildpack-examples/cache-image-example
docker inspect localhost:5000/buildpack-examples/cache-image-example

Now let’s take a look at the cache image -

docker pull localhost:5000/buildpack-examples/maven-cache-image:latest
docker inspect localhost:5000/buildpack-examples/maven-cache-image:latest

The cache image we produced may now be used by builds on other machines. Note these builds may also update the specified cache-image.

The following command will restore data for the samples/java-maven:maven_m2 layer from the cache image.

pack build localhost:5000/buildpack-examples/second-cache-image-example \
    --builder cnbs/sample-builder:jammy \
    --buildpack samples/java-maven \
    --path samples/apps/java-maven \
    --cache-image localhost:5000/buildpack-examples/maven-cache-image:latest \
    --network host \
    --publish

Image Retention

Managing the lifecycle of images should be the responsibility of the owner, as the platform does not automatically clean up old images from the registry.
You can refer to your registry’s documentation to learn how to accomplish this.

• AWS ECR users can find information on how to delete images in the AWS ECR documentation, specifically in the section on image deletion.
• Docker users can consult the Docker documentation on Advanced Image Management to find out how to delete images.

---