The `digest` data type, used throughout docker for image verification
and identity, has been broken out into `opencontainers/go-digest`. This
PR updates the dependencies and moves uses over to the new type.
Signed-off-by: Stephen J Day <stephen.day@docker.com>
This extract what was in registry_test.go and
registry_mock_test.go. This also move `RegistryHosting`
requirement to `registry.Hosting`
Signed-off-by: Vincent Demeester <vincent@sbr.pm>
