Caching for docker-in-docker builds

I've followed the advanced example in the container registry announcement to set up a CI build of a docker image. Unfortunately the image takes a very long time to build, and I cannot enable caching, since /var/lib/docker is in the linked docker:dind service image, not in the build image.

The release-image job is also missing a docker pull step. That job gets a fresh docker:dind service, so it doesn't have the newly built image. The same is probably also true of the two test jobs.