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

arkindex/manage.py

@@ -2,7 +2,8 @@
 import os
 import sys
 
-if __name__ == "__main__":
+
+def main():
     os.environ.setdefault("DJANGO_SETTINGS_MODULE", "arkindex.project.settings")
     try:
         from django.core.management import execute_from_command_line
@@ -20,3 +21,8 @@ if __name__ == "__main__":
             )
         raise
     execute_from_command_line(sys.argv)
+
+
+if __name__ == "__main__":
+    sys.stderr.write("WARNING: manage.py is deprecated, you should use the `arkindex` script instead\n")
+    main()