Build runtime Docker images.

Also added release process to push these images to GCS.

Notes:
* Currently this uses jbeda/docker-registry instead of google/docker-registry.  I can change it over when https://github.com/GoogleCloudPlatform/docker-registry/pull/9 is in.
* Currently we only upload docker images to GCS.  This is to enable fast testing on GCE.  There should be a mode/option to upload to the docker index instead.