pushV2Tag already deduplicates layers, but the scope of this
deduplication is only for a particular tag. If we are pushing all tags
in a repository, we may check layers several times. Fix this by moving
the layersSeen map from the pushV2Tag function to the v2Pusher struct.
In addition to avoiding some useless round-trips, this makes the "docker
push" output less confusing. It formerly could contain many repeated
lines like:
124e2127157f: Image already exists
124e2127157f: Image already exists
...
Add test coverage based on the "docker push" output: a hash should not
appear multiple times when pushing multiple tags.
Fixes#14873
Signed-off-by: Aaron Lehmann <aaron.lehmann@docker.com>
