re-add gunicorn to core docker requirements
re-add gunicorn to core docker requirements

(oops, broke the demo server)

SchrodingersGat/InvenTree,SchrodingersGat/InvenTree,inventree/InvenTree,inventree/InvenTree,SchrodingersGat/InvenTree,SchrodingersGat/InvenTree,inventree/InvenTree,inventree/InvenTree