WIP: Add support for personal repo

Set up support for personal repositories to make it easy for people to play with this code without needing access to the canonical Docker repository. Follows the guide at https://docs.tilt.dev/personal_registry.html.

WIP because, in local testing, it seems to ignore the override repo. My theory is that it's happening because Bazel's container_image has already published the image before Tilt gets a chance to do any rewriting. I might have to thread the personal registry location through Bazel as a workspace rule (which would be a lot less painful if https://github.com/bazelbuild/bazel/issues/3732 worked).