Fix CI error

The error is due to the docker image. This changes the image to one that works and seems to be better maintained.

.gitlab-ci.yml

@@ -6,6 +6,8 @@ stages:
 
 build:
     stage: build
+    before_script:
+    - apk add py3-pygments
     script:
     - cd spec
     - lualatex -shell-escape -recorder '\def\nocomments{}\input{doc.tex}'