Review and enhance Docker setup

- [x] Verify the existing `Dockerfile` builds and runs the service correctly.
- [x] Ensure the container accepts env vars from `.env` / `.env.defaults`.
- [x] Create/update `docker-compose.yml` including the service + PostgreSQL for local dev. (If needed)
- [x] Add healthchecks in `docker-compose` (service and Postgres). (If needed)
- [x] Document `docker-compose up` workflow in the README (commands, ports, envs).

Start by checking the Dockerfile, docker-compose.yml, .env, .env.defaults, and README against the completed checklist. Verify the build, service and PostgreSQL startup, environment variables, healthchecks, ports, and documented docker-compose up workflow; done means each listed item works for local development.

Written by the indexing model from the issue text.