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)
(cherry picked from commit 33984f256b)
Signed-off-by: Tibor Vass <tibor@docker.com>