Docker-based CI

This is based off of @yookoala's work in #99

I rebased it on the latest master, cleaned it up a bit, and made some modifications:

• I removed the local testing, as there's really no point
• I parametrized the FROM line so that we have a single Dockerfile (as already was the case for the original pull request) but in a way we can still have automatic builders in the Docker Hub

That last point is a big unknown: I have no clue whether it works at all. :D

However, this suggests it should: https://github.com/docker/hub-feedback/issues/508#issuecomment-222520720