A statically hosted registry that responds correctly to GET with a
manifest will load the right digest (by looking at the manifest body and
calculating the digest). If the registry returns a HEAD without
`Docker-Content-Digest`, then the client Tags().Get() call will return
an empty digest.
This commit changes the client to fallback to loading the tag via GET if
the `Docker-Content-Digest` header is not set.
Signed-off-by: Clayton Coleman <ccoleman@redhat.com>
a2015272c1