Update mkdocs-gitlab-pages.gitlab-ci.yml

mkdocs-gitlab-pages.gitlab-ci.yml

-.job-image: &job-image docker.io/library/python:3.9-slim-bullseye
+.job-image: &job-image registry.cern.ch/docker.io/library/python:3.9-slim-bullseye
 
 .job-script: &job-script
   - apt update && apt install -y git subversion