Optimize the CI builds

[k0d]

I'm opening this issue so that we can discuss how we speed up the builds. Here are a few ideas.

Docker image containing dependencies
Clean gcc-arm-toolchain folder so it contains only needed files (my initial tests show ~600mb can shrink to ~100mb).
Use GitHub Actions caching (https://help.github.com/en/actions/configuring-and-managing-workflows/caching-dependencies-to-speed-up-workflows#using-the-cache-action)

[microdev1]

We now implement caching in most places and I don't think that there is a need for using a docker image in CI now.
We can still however publish a docker image with all the CP deps as a package.

[tannewt]

Thanks @microdev1. I'll close this.