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

.gitea/workflows/docker-build.yml

@@ -13,20 +13,20 @@ jobs:
         pg_version: [15, 16, 17, 18]
     steps:
       - name: 📥 Checkout code
-        uses: actions/checkout@v4
+        uses: https://github.com/actions/checkout@v4
 
       - name: 🛠️ Set up Docker Buildx
-        uses: docker/setup-buildx-action@v3
+        uses: https://github.com/docker/setup-buildx-action@v3
 
       - name: 🔐 Login to Gitea Registry
-        uses: docker/login-action@v3
+        uses: https://github.com/docker/login-action@v3
         with:
           registry: gitea.killinger.fr
           username: maxime.killinger
           password: ${{ secrets.DOCKER_TOKEN }}
 
       - name: 📦 Build Docker image
-        uses: docker/build-push-action@v5
+        uses: https://github.com/docker/build-push-action@v5
         with:
           context: .
           load: true
@@ -74,7 +74,7 @@ jobs:
         run: docker rm -f test-pg${{ matrix.pg_version }} || true
 
       - name: 🚀 Push Docker image
-        uses: docker/build-push-action@v5
+        uses: https://github.com/docker/build-push-action@v5
         with:
           context: .
           push: true
@@ -85,7 +85,7 @@ jobs:
 
       - name: 🏷️ Tag latest (PG18 only)
         if: matrix.pg_version == 18
-        uses: docker/build-push-action@v5
+        uses: https://github.com/docker/build-push-action@v5
         with:
           context: .
           push: true