Improve GCR Tag State Correction

[skeet70]

#5 added working tag state correction but there are improvements we'd like to make if we need it again.

• Pull down the buildlog SHA-d images and do all tagging and manifest creation locally.
  • that would let us print out a dry run example of what will be done. Then after a confirmation step a pass to statically add the remote to all names/tags could be done, and pushed.
  • one blocker to look out for is if a local manifest pointing to local images can be re-tagged/named and pushed up, or if we need to create manifests after pushing to remotes, or something in between.