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

docker-entrypoint-wrapper.sh

@@ -0,0 +1,14 @@
+#!/bin/bash
+set -e
+
+# Fix permissions for mounted volumes (PG18+ requirement)
+# This runs as root before switching to postgres user
+if [ "$(id -u)" = '0' ]; then
+    # Ensure data directory exists with correct permissions
+    mkdir -p /var/lib/postgresql
+    chown -R postgres:postgres /var/lib/postgresql
+    chmod 700 /var/lib/postgresql
+fi
+
+# Execute the original entrypoint
+exec docker-entrypoint.sh "$@"