Wait for a dependency to report healthy before starting a Docker Compose service using depends_on service_healthy

Documented steps

1. Give the dependency service a `healthcheck` block (e.g. for mysql: `test: ["CMD", "mysqladmin", "ping", "-h", "localhost"]`, with interval/timeout/retries).
2. In the dependent service use the long-form `depends_on` with `condition: service_healthy` under the dependency name.
3. Example: app: { depends_on: { db: { condition: service_healthy } } }.
4. Run `docker compose up`; Compose starts the app only after the dependency is reported healthy.
5. Official docs: https://docs.docker.com/compose/how-tos/depends_on/