The list-form environment syntax in the README passed quote characters
through as part of the value, so LIBRARY_ID was read as '"<id>"' and no
matching library was found. Switch to mapping syntax, where YAML strips
the quotes.
- Pin the example to a versioned image instead of implicit latest
- Add a checked-in compose.example.yaml
- Validate it with `docker compose config` in CI
- Link the Docker Hub image from the README
