CI: Use own images to speed up jobs

• First try, please review and criticize.
• To make the jobs even faster, I could remove the calls to apt etc. completely; one would then need to update the corresponding Dockerfiles. I guess this would be acceptable, as the stuff changes quite rarely. Running apt etc. each time seems like a waste of time and energy.
• I've used debian:unstable for the linting image, which pulls in a newer pylint version. The corresponding job bails out due to R1705 -- there is a lot of controversy on the Internets about this rule, enforcing vs. disabling it -- please tell me what you prefer and I'll adapt accordingly.
• I've granted you access rights to the repo which currently builds the Docker images. We could also transfer the repo to your namespace, in case you would like to. Please keep the repo access private, as a privileged runner is connected to it, which might result in privilege escalation if "bad code" is pushed.
• Also, we could connect the runner (in unprivileged) mode to this repo, as the cache is bigger and stored longer (in contrast to the public runners), which would possibly decrease the job execution times even further.

Closes #45 (closed)