Publish an Heptapod base Docker image

This is the logical follow-up to #1 (closed): once we are able to produce a Debian package from the Heptapod Rails sources, use it to produce a Docker image and upload it to Docker Hub as. e.g., heptapod/build-base.

This image would not be viable for deployments, as it would not even have Mercurial, but it would be usable as a replacement for the GitLab image in the Heptapod Docker builds.