fix: prestart.sh loaddata re-runs on every container restart (#1502)

deploy/docker/prestart.sh

@@ -6,7 +6,7 @@ ADMIN_PASSWORD=${ADMIN_PASSWORD:-$RANDOM_ADMIN_PASS}
 if [ X"$ENABLE_MIGRATIONS" = X"yes" ]; then
     echo "Running migrations service"
     python manage.py migrate
-    EXISTING_INSTALLATION=`echo "from users.models import User; print(User.objects.exists())" |python manage.py shell`
+    EXISTING_INSTALLATION=`echo "from users.models import User; print(User.objects.exists())" |python manage.py shell 2>/dev/null | tail -1`
     if [ "$EXISTING_INSTALLATION" = "True" ]; then
         echo "Loaddata has already run"
     else