They opened sourced the runner[0] if you're interested in learning how it works. Understanding the internals of it may or may not help the syntax and concepts of Actions land though.

My guess is that it is unlikely to see your request for a more generalized script or Dockerfile runner realized because that (Dockerfiles) was the original implementation of Actions during the beta; they pivoted away from that to the current form.

[0] - https://github.com/actions/runner