Update .gitlab-ci.yml

.gitlab-ci.yml

-image: python:3.8
+image: gitlab.labranet.jamk.fi:4567/tools-and-services/mkdocs-plantuml-docker
+
 pages:
     script:
-        - apt update && apt install git -y
         - pip install mkdocs
         - pip install mkdocs-material
-        - pip install plantuml-markdown
-        - git clone https://github.com/g-provost/lightgallery-markdown.git
-        - cd lightgallery-markdown
-        - python setup.py install
-        - cd ..
         - mkdocs --version
         - mkdocs build
     artifacts: