fix docker build

Dockerfile

@@ -26,6 +26,7 @@ RUN DATABASE_URL=postgres://x/x \
     DEFAULT_CONTRACTEE_ZIP=x \
     DEFAULT_CONTRACTEE_DISTRICT=x \
     DEFAULT_CONTRACTEE_ICO_NUMBER=x \
+    DEFAULT_STAFF_GROUP=x \
     python manage.py collectstatic --noinput --settings=registry.settings.production
 
 RUN bash -c "adduser --disabled-login --quiet --gecos app app &&  \