feat(docker/production): create superuser in initial setup

We add one step to the master entrypoint script to ensure a
superuser exists with username and password configured from the
environment variables DJANGO_SUPERUSER_USERNAME and
DJANGO_SUPERUSER_PASSWORD. The step does nothing if these variables
are not set or the user already exists.

docker/production/entrypoint-master.sh

@@ -16,6 +16,7 @@ if ! [ -f completed_initial_run ]; then
     python jdav_web/manage.py compilemessages --locale de
 
     python jdav_web/manage.py migrate
+    python jdav_web/manage.py ensuresuperuser
 
     touch completed_initial_run
 fi