docs: improve contributor testing guide and dockerised cookstyle

[Firefishy]

• Rewrite CONTRIBUTING.md with step-by-step local setup and test workflow (Cookstyle + Test Kitchen)
• Document where Kitchen suites and InSpec tests live and how to debug failed converges
• Add "Need help?" pointing to #osmf-operations (IRC/Matrix bridge) and align README contact info
• Update Dockerfile to Debian 13 (ruby:3.3-trixie) and add docker-compose.yml for running Cookstyle.

[Firefishy]

I believe the debug step is not ok: docker exec -it 9a7e6edc5b-dns-ubuntu-2204 bash -l This commands expects the running container id, not the image.

That is the running container id as per docker ps.

[Firefishy]

As documented it works. Name or ID are both acceptable input.

[mmd-osm]

Apologies, you're of course right. I somehow missed the container name, as it was hiding behind a scrollbar. 😔