support cache between runs

Currently, any --mount=type=cache mounts in the Containerfile are ineffective because the gitlab runners are not persisting that buildah cache between runs.

This could be supported at a few different levels:

• buildah's distributed cache with --cache-from/--cache-to
• gitlab-ci job cache configuration
• assuming gitlab runners use the docker executor, use its cache volume

distributed cache

Pros:

• should be already doable by adding --cache-from and --cache-to to the existing CONTAINER_BUILD_EXTRA_ARGS (though we could make this more convenient)
• best option for when build jobs are distributed between many different runners

Cons:

• requires more bandwidth if your runners are not near your container registry

gitlab-ci.yml job cache configuration

Pros:

• the most explicit and familiar to developers who do other types of gitlab-ci builds

Cons:

• probably is going to pack everything up in a zip file, which seems unnecessarily slow for large images

executor volumes

Pros:

• works the most like you'd expect from running podman or buildah locally
• least archive and network overhead

Cons:

• assumes something about the runner's configuration
• not useful when your job switches runners