You have 2 options.
-
Use a different Hugo Docker image on Gitlab that is more recent.
-
Choose the Hugo version you need on Gitlab.
EDIT
Upon closer inspection of your .gitlab-ci.yml
you are using the latest Hugo at the Gitlab container registry and that is version 0.30.2
Hugo 0.31.1 does not exist in the Gitlab container registry yet: https://gitlab.com/pages/hugo/container_registry
The hugo/monachus image is also v. 0.30.2 v. 0.31.1 see here