chore: switch from gunicorn to Django development server in dockerfile

Replace gunicorn production server with Django's development server
(manage.py runserver) for easier debugging. The gunicorn configuration
has been commented out rather than removed for easy restoration.

**Warning:** This change makes the application unsuitable for production
deployment as runserver is not designed for production use.

dockerfile

@@ -29,4 +29,5 @@ EXPOSE 8000
 CMD \
   python manage.py migrate && \
   python manage.py collectstatic --noinput && \
-  gunicorn config.wsgi:application --bind 0.0.0.0:8000 --workers 3
+  python manage.py runserver 0.0.0.0:8000 && \
+#   gunicorn config.wsgi:application --bind 0.0.0.0:8000 --workers 3