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

Part of the problem is that Dockerfiles are not pure image-building recipes. The ability for Dockerfiles to depend on other Dockerfiles would create a way to express such overrides. That's a feature that has been requested from Docker. Triggering cascading builds is another issue.