Convert the docker-based GHA into a composite action

[webknjaz]

I think it would be reasonable to refactor the GHA to use a better action type. Docker is limited to Ubuntu workers and has a certain amount of runtime overhead that I believe can be addressed with Composite GitHub Actions. I've promptly checked the current setup and I don't believe there's anything that is tightly coupled with the docker runtime and can't be substituted.

Ref: https://docs.github.com/en/actions/creating-actions/creating-a-composite-action

[platisd]

I also think it'd work and would improve the run times.
Feel free to submit a PR! 👍

[webknjaz]

Here's an example of using gitlint instead of this action: https://github.com/cherrypy/cheroot/pull/579/files. Maybe somebody interested in this will get some inspiration from that patch. I'm moving away from using this action for now.