Fix Docker image to run in healthy mode

Refs https://redmine.teklia.com/issues/6470

• Use django-admin in Docker for easier deployments, as it's easier to setup worker commands or any other django-admin command than relying on ./manage.py
• Update healthcheck to verify /api/v1/corpus instead of /api/v1/public-key that is dependant on ponos
• Use $PORT in dockerfile
• Use arkindex script in dockerfile
• Warn on usage of ./manage.py
• Expose arkindex for developers too