Subject names for attestations

[jedevc]

Follow up to #2935.

From conversation with @tonistiigi:

thinking about this bit more, I think we should put the repo name here. We should at least have something that says that the digest points to an image (when we add attestation to local output then it points to a file and not an image)

SGTM :+1: - this makes sense from the buildkit side, since pushed images always have a name.