chore: lint and update docker-compose.yml

- Fixed port variable interpolation to use ${PORT:-8000} for a default value.
- Updated volume configuration to use a named volume (guestbook_data) mounted at /data.
- Improved YAML formatting for clarity.

.env

@@ -0,0 +1,11 @@
+# Port to expose (default 8000)
+PORT=5000
+# Flask environment setting (production)
+FLASK_ENV=production
+# Path to the SQLite database (this file will be stored in the mounted /data volume)
+DATABASE_PATH=/data/scripts/guestbook.db
+# Number of Gunicorn workers (adjust as needed)
+GUNICORN_WORKERS=3
+PID=1000
+GID=1000
+