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

README.md

@@ -23,12 +23,19 @@ Docker image combining **PostgreSQL** with three powerful extensions:
 docker run -d \
   --name my-postgres \
   -e POSTGRES_PASSWORD=mysecretpassword \
+  -v postgres_data:/var/lib/postgresql \
   -p 5432:5432 \
   gitea.killinger.fr/maxime.killinger/postgres-ts-vectors:latest
 ```
 
 All extensions are **automatically created** on first startup.
 
+## Data Persistence
+
+> [!IMPORTANT]
+> Starting with PostgreSQL 18, mount volumes at `/var/lib/postgresql` (not `/var/lib/postgresql/data`).
+> Data will be stored in a versioned subdirectory (e.g., `/var/lib/postgresql/18/main`), which facilitates future upgrades with `pg_upgrade`.
+
 ## Environment Variables
 
 | Variable | Description | Default |