PostgreSQL 18+ Docker images changed the data directory structure to use
versioned subdirectories (e.g., /var/lib/postgresql/18/main) instead of
the traditional /var/lib/postgresql/data path.
Changes:
- Dockerfile: Add mkdir and chown to ensure /var/lib/postgresql has
correct permissions for the postgres user before volume mount
- README.md: Update Quick Start with volume mount example
- README.md: Add 'Data Persistence' section explaining PG18+ requirements
This fixes the 'mkdir: cannot create directory: Permission denied' error
that occurred when mounting volumes at /var/lib/postgresql.
See: https://github.com/docker-library/postgres/pull/1259
