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>
This commit adds the `docker-image` attribute to the main Nix
entrypoint, invoking `nix/pkgs/docker-image` which will build
a 'smart' docker image that can load keyfiles or a pier and
boot a ship
It includes a README for the official docker image, suitable
for posting as the README to a Docker Hub or similar docker
image repository.
