add: custom postgress data path for coolify docker compose

DOCKER.md

@@ -39,6 +39,15 @@ Required:
 - `NODE_ENV` - Set to `production`
 - `PORT` - Default `3000`
 
+Optional (Docker Compose):
+- `POSTGRES_USER` - Database user (default: `wishlistuser`)
+- `POSTGRES_PASSWORD` - Database password (default: `wishlistpassword`)
+- `POSTGRES_DB` - Database name (default: `wishlist`)
+- `POSTGRES_DATA_PATH` - Custom path for PostgreSQL data
+  - Example: `/mnt/storage/wishlist/postgres`
+  - Must be an absolute path
+  - Directory must exist with proper permissions
+
 ## Database Setup
 
 ```bash