Drop best-effort Docker setup

[chalin]

• Fixes Example "docker compose --build" Fails on Mac #453
• Why: the Docker setup was best-effort (not officially supported, not exercised by CI -- see README: defer to the Docsy user guide #488) and kept attracting maintenance issues; Docsy is officially dropping Docker support (Remove broken Docker config and retire Docker docs docsy#2748).
• Removes the Dockerfile, docker-compose.yaml, the README "Running a container locally" section, and the CONTRIBUTING Docker notes (Renovate config needs no change; its Docker manager simply finds no files).
• Merge order: land Remove broken Docker config and retire Docker docs docsy#2748 first -- until it does, the live Docker quickstart still tells users this repo provides the Dockerfile being deleted here.

Validated: build, test:site, and check:links green (these cover the built site, not the README/CONTRIBUTING edits); the doc edits verified by a tracked-file sweep (no Docker/Compose/DOCSY_USER/quickstart-URL references remain) and prettier.