This mimics the behavior of `docker-entrypoint.sh` before it starts the PostgreSQL server.
It has three main goals/uses:
1. (most importantly) as an example of how to use "docker-entrypoint.sh" to extend/reuse the initialization behavior
2. ("docker-ensure-initdb.sh") as a Kubernetes "init container" to ensure the provided database directory is initialized; see also "startup probes" for an alternative solution
(no-op if database is already initialized)
3. ("docker-enforce-initdb.sh") as part of CI to ensure the database is fully initialized before use
(error if database is already initialized)
