Adds a Release Channels section to the main README explaining the three
Docker image tracks users can choose from: stable :latest (Docker Hub),
nightly :dev (GHCR, rebuilt from dev branch), and pinned version tags.
Includes a decision table for picking the right channel and switching
instructions for docker-compose users.
Notes on the Unraid section that the template points at :latest by
default, and how to switch an Unraid container to the :dev channel
by editing the Repository field.
Adds a Contributing section covering the dev → main PR workflow, how to
branch off dev, expectations around ruff + pytest passing locally, and
how to run the dev gunicorn config.
Mirrors a short release-channels blurb at the top of
Support/README-Docker.md pointing at the main README's full guide.
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