Fix and validate Compose quick start

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

.github/workflows/docker-publish.yml

@@ -16,6 +16,9 @@ jobs:
     steps:
       - uses: actions/checkout@v2
 
+      - name: Validate compose example
+        run: docker compose --file compose.example.yaml config --quiet
+
       - name: Run tests
         run: |
           if [ -f docker-compose.test.yml ]; then