Cache docker layers to reduce build time

[TheAspens]

From @ChristianBeer

I looked into caching the docker layers and we would gain some time but this doesn't look trivial with the ansible setup we use. Right now we are still under 10 minutes runtime where the actual tests are only several seconds. So adding more tests does not add too much a burden and we are still far away from the 30 minute limit.

[marius311]

Yea, I don't see any super easy way of doing it either. FWIW, once some of the cool experimental stuff in moby/moby#32507 drops, build caching will probably be really easy on the Docker side, so I'd say wait until we hit the 30min limit, then decide what to do based on if that stuff is available yet or not.

Btw, I'm not following super closely here but don't hesitate to ping me directly with any questions or requests re: this stuff ;)