Create common interface for docker daemon and docker registry

[MeneDev]

The interface should allow to query all tags and their digest.
Blocked by #1 and #2

[MeneDev]

I created a first draft in dockref.Repository also instead of returning strings ImageNameProcessor now returns (dockref.Reference, error) and dockref.Reference contains it's format which allows toggling different components (e.g. tag).

While the interface contains only one function Resolve(reference Reference) (Reference, error) it might be enough as a first step, allowing to resolve e.g. nginx to something like nginx:1.2@sha256….