Add an ImageSource implementation to docker.go
Add an ImageSource implementation to docker.go

The ImageSource type does not provide all of the functionality of
docker.go, but we will be able to reuse the ImageSource parts in an
OpenShift client.

This is only a restructuring, does not change behavior.

masters-of-cats/image,projectatomic/skopeo,mikebrow/skopeo,mtrmac/skopeo,jwhonce/skopeo,projectatomic/skopeo,jwhonce/skopeo,containers/image,mikebrow/skopeo,mtrmac/skopeo,mtrmac/skopeo,containers/image