Add cache cleanup option to local cache exporter

[MarcelCoding]

Implemented an option to remove old versions of a newly created image and images older than a specified time.

Reason/Usecase: docker/build-push-action#252
My temporarily solution: docker/build-push-action#252 (comment)

[tonistiigi]

BuildKit doesn't have an image store. For cleaning up build cache there is buildctl prune (docker buildx prune)

[crazy-max]

@tonistiigi It's about the local cache exporter. Atm caches are copied over the existing cache so it keeps growing (#1850).

[CMonoceros]

Is there any progress on this option?

[Niek]

docker/buildx#535 has been merged now, I think #1974 is still WIP.