fix(docker): resolve permission denied error for PG18+ volume mounts

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

Dockerfile

@@ -46,6 +46,9 @@ COPY --from=builder /tmp/timescaledb/usr/share/postgresql/ /usr/share/postgresql
 COPY init-extensions.sh /docker-entrypoint-initdb.d/
 RUN chmod +x /docker-entrypoint-initdb.d/init-extensions.sh
 
+# Ensure correct permissions for data directory (important for PG18+ with volume mounts)
+RUN mkdir -p /var/lib/postgresql && chown -R postgres:postgres /var/lib/postgresql
+
 # Healthcheck
 HEALTHCHECK --interval=30s --timeout=5s --start-period=60s --retries=3 \
     CMD pg_isready -U postgres || exit 1