Container manifests can define docker registry

- Allows for each image to be pulled from a custom docker registry
- ContainerManifest can specify authentication for a private registry
- No breaking changes
The way I understood this, a `Container` is tied to a single image. The same image can be available on more than one Registry, and therefore I decided to keep the Registry struct separate from the `image string`. 

I'm not aware of how images can be specified with registry baked in.