Curious why --cache-to creates an image index

[mxmeinhold]

I noticed that docker buildx build --cache-to creates a application/vnd.oci.image.index.v1+json at the cache destination.

This struck me as odd, given that the "manifests" referenced by it seem to be pushed as blobs in my testing, and aren't resolvable as manifests in the registry. This results in some broken expectations for client (e.g. crane config tries to fetch one of the "manifests" as a manifest, which of course fails). I would think this could fairly easily translated to an image manifest, since the application/vnd.buildkit.cacheconfig.v0 seems a good candidate for the config field, and the other entries look to already be layer mediatypes.

Is there some restriction that requires cache be pushed as an image index?

Context:

$ docker buildx version
github.com/docker/buildx v0.5.1-docker 11057da37336192bfc57d81e02359ba7ba848e4a

I'm testing with crane version 0.7.0 and manually inspecting resources that --cache-to pushed to a self hosted distribution spec compliant registry using logs and curl.