Always attempt to add digest even when tag already exists. Ensure digest does not currently exist. When image id is mismatched, output an error log. Signed-off-by: Daniel Nephin <dnephin@docker.com> Signed-off-by: Derek McGowan <derek@mcgstyle.net> (github: dmcgowan) Upstream-commit: 33984f256b1a281b1130ac7e8edb7bc311750ccf Component: engine