Proposal: Initial images proposal

This is a proposal to add a higher level image component that can keep track of Docker image repositories, images, and image metadata.

This is not about reinventing how Docker handles images or an attempt to make Kubernetes also a Docker registry; instead, we're hoping to facilitate concepts such as:

- a virtual unified view of image repositories potentially spanning multiple registries
- reacting to image changes to perform a variety of actions, such as downstream builds or new deployments
- allowing users to override default image metadata consistently when an image repository changes
- dealing with Docker image metadata in a time series, for the purposes of making images easily consumable by other components of a deployment system

Suggest global, not project.   Projects that want to use the actual docker image depot should be able to do that by default without requiring creation of a per project ImageRepository object.