fix(docker): move Postgres volume mount to /var/lib/postgresql for 18+

[bigpuritz]

Summary

Postgres 18 image rejects the previous /var/lib/postgresql/data mount layout, so docker compose up -d from the repo root fails — breaking the quick-start documented in the root README.md.

This PR mirrors the upstream fix from plugwerk/plugwerk#293 (commit cb00cfe): mount the volume at /var/lib/postgresql instead, with the same explanatory comment pointing at docker-library/postgres#1259.

The volume name (pgdata) is intentionally kept to avoid an unnecessary rename for existing local volumes.

Diff

```diff
volumes:

•  - pgdata:/var/lib/postgresql/data
  

•  # postgres:18+ requires the volume at /var/lib/postgresql (not /var/lib/postgresql/data)
  

•  # so PGDATA can live in a major-version-specific subdirectory alongside config.
  

•  # See https://github.com/docker-library/postgres/pull/1259.
  

•  - pgdata:/var/lib/postgresql
  

```

Migration note for existing local volumes

Any `examples_pgdata` volume created before this fix is unusable with Postgres 18 and should be removed once:

```bash
docker compose down -v # or: docker volume rm examples_pgdata
```

No data loss concern — the previous setup never successfully ran with PG 18, so the volume only contains demo data from earlier PG versions, if anything.

Test plan

• `docker compose up -d` on an empty volume → `examples-postgres-1 Healthy`, server `/actuator/health` returns `{"status":"UP"}`.
• `docker exec examples-postgres-1 ls /var/lib/postgresql/` shows the expected `18/` subdir.
• `docker compose down` (no `-v`) followed by `docker compose up -d` → data persists, server comes back up cleanly.

Closes #8
Relates to plugwerk/plugwerk#293