Support docker containers

[baolsen]

Issue
The runner should be updated to prefer steps running inside docker containers.

Running steps inside docker containers is already possible (and works well) with the runner, it just isn't clarified and needs a little work.

This ensures a clean execution environment for each job.

This also means removing the installation of unnecessary software packs, and rather have the user specify what to install, apart from docker (BREAKING CHANGE).

Generally there should be a compelling reason to install software directly on the runner, vs in a docker container image.

It would be good to have an example of running steps in docker containers.

Housekeeping

Docker can fill the instance storage, and this can be cleaned by periodically running:

docker system prune -af