Building docker images takes too long

[slinkydeveloper]

Essentially I see two issues here:

• CI builds on PRs take too long. Few months ago I remember there was an effort to optimize them, but now it takes again 20 mins. Not sure exactly what changed in the meantime that slowed it down dramatically.
• At least on my local machine, just docker-debug takes more or less the same amount of time of just docker, and I'm not sure it's reusing caching or not. This impacts the ability to perform local testing.

We either find two separate solutions for the two issues, or a solution that works for both.

I brainstormed few ideas with @tillrohrmann, and one option to explore could be the following: some new image build command like just docker-local, that basically builds the runtime without docker, and afterward it copy pastes the built executable in a scratch container image. This would not need cross compilation (neither locally, neither on CI). For this solution to work, most likely the runtime needs to be built statically linked with musl, to make sure it works on the scratch image.