Fetch the build image from the registry instead of building it on the fly.

Rather than having slaves build Docker images on the fly to perform continuous integration, a single pre-built image is now stored inside the registry. This is the same kind of setup used for the continuous integration of Coq.

As a consequence, slaves now have to provide docker executors rather than shell ones. That said, we still need a shell slave around to pre-build the image, as well as to perform the deployment. (Eventually, this should be changed to a docker-in-docker executor.)

Benefits:

• The Dockerfile description can now be modified as needed.
• Slaves should now be a bit more secure (at least the docker ones).
• We might be able to reduce the storage space needed by the slaves. (We are getting dangerously close to our 400GB quota.)

Downsides:

• Testing a new OCaml compiler or an additional Opam package is no longer a matter of committing a few lines and waiting a few minutes.
• Pre-building the Docker image has to be triggered manually.
• It takes about two hours to create the image.