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

@@ -46,11 +46,14 @@ 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
 
+# Add entrypoint wrapper to fix volume permissions at runtime
+COPY docker-entrypoint-wrapper.sh /usr/local/bin/
+RUN chmod +x /usr/local/bin/docker-entrypoint-wrapper.sh
+
 # Healthcheck
 HEALTHCHECK --interval=30s --timeout=5s --start-period=60s --retries=3 \
     CMD pg_isready -U postgres || exit 1
 
-USER postgres
-
-# Preload required libraries
+# Run as root initially, wrapper will fix permissions then switch to postgres
+ENTRYPOINT ["docker-entrypoint-wrapper.sh"]
 CMD ["postgres", "-c", "shared_preload_libraries=timescaledb,vchord"]