cache docker layers, build binary artifacts, use docker in testing

Cache docker layers from previous master branch docker images, thus avoiding costly rebuilds when dependencies do not change.

Changelog entry

• changelog entry included in commits (often not a good idea!)

TODO

• proof of concept: !550 (merged).
• should we adjust the actual tests to use docker in this MR?
• ...or adjust tests to use docker in a future MR?
• CI builds windows binary artifact (we need our own runner for mac)
• CI builds gnu/linux binary artifact
• defer these for future MR:
  • use requirements.txt #941 (closed)
  • use ubuntu 18.04 as base: #942 (closed)
• switch the branch generating :latest to master before merge! (this is explained a bit in !550 (merged)).