Hi,

There's a couple of issues here about some registries not supporting --export-cache of type=registry and I'm experiencing the same thing with JFrog Artifactory.

So far I've gathered that this is likely because of the special application/vnd.buildkit.cacheconfig.v0 media type being pushed, which doesn't appear to be a part of either the Docker (v2 schema 2) spec or the OCI spec.

Is there a plan for this becoming part of OCI or some other standard?

Correct me if I'm wrong please, but when it comes to the usefulness of buildkit caching (and therefore buildkit in general for many people), it is strange to me that the functionality depends on this buildkit-specific media type, which, because of not being part of any of the specs, is unlikely to be generally supported by registries.

Additionally, I think it would be really useful to mention this in some way in the docs and perhaps track common registry provider support.

Since this is something I'm running into as a mere mortal container user who just wants to be able to efficiently build containers (rootless in Kubernetes, which buildkit seems to be the best fit for right now), there's a chance I'm missing something fundamental here, so please do feel free to educate me :)

Thanks in advance!