d3c5aa74c1 introduced a Docker image intended for deploying Urbit ships
This commit modifies the build.yml GitHub Actions workflow to build the
docker image as part of the push/PR workflow, and adds a
`release-docker.yml` workflow, which runs on any tag push `urbit-v*` and
pushes the image to Docker Hub, also synchronizing the README.
Co-authored-by: Josh Lehman <josh@urbit.org>
