LocalImage should return a DigestIdentifier if available

[ekcasey]

Given a LocalImage created from a tag or digest reference, the returned Identifier should be a DigestIdentifier if the docker inspect contains a RepoDigest with repository matching the repository from the original requested reference.

Note While implementing this we might want to improve the workflow for fetching identifiers. Identifiers should only be available for saved images, not those that have been mutated but not yet commited.