Make it possible to overwrite image TS with label
This adds support for overwriting the image created at timestamp with
labels which are set during build. Supported labels (for now) are the
Open Container Image (OCI) spec and the (legacy) Label Schema (LS)
We prioritize OCI over LS, with a fallback to the CreatedAt.
This should serve a wide range of users who either have internal rules
for image builds (e.g. timestamp set to null as reported in #1797) or
want a stable reliable pointer they can have control over (#746, #891).