Accelerate CI, extend docker build

[MischaPanch]

Most time in the CI is spent installing dependencies. We already use caching there, but likely not perfectly.

We also only build a minimal docker image that doesn't include all dependencies.

It would be useful to also build a maximal docker image that includes all preparatory steps from the CI workflow. In this image, all tests for all languages would run out of the box without further installations. We could then use this image for CI tests, though since we also test on windows and macOS it's likely not enough and we'd need to use some other strategy to not have to install so much from scratch all the time.

Help from the community on this would be appreciated, ideally by someone with experience on how to optimize GitHub CI workflows.

Note that making the maximal docker image is not completely trivial, in the current CI workflow we use a lot of setup actions from the GH action marketplace which would need to be translated into appropriate docker commands.

[vbhavh]

/assign

[MischaPanch]

Thanks @vbhavh !

[MischaPanch]

NB: for those LS that download things on the fly, starting a language server downloads the dependencies.

It's not urgent and maybe not even beneficial to include such dependencies in the docker image though.