docker

In this change we modify the CI pipeline to use pre-built Docker images
for both build and deploy stages.  To this end, we create
`docker/{build-env,deploy-env}`, two directories containing Dockerfiles
describing how to build those images, and scripts to push them to a
Docker registry.

Currently, docker.card10.badge.events.ccc.de seems broken. Thus, we push
to the Warsaw Hackerspace Docker registry.  Since the Docker images will
be pulled rarely anyway (as they are cached by build nodes), any low
bandwidth-caused slowdowns will not be usually end up in the CI critical
path.

We also bump `.gitlab-ci.yml` to point to those new images.