Fixed CI build

.gitlab-ci-docs.yml

@@ -2,9 +2,12 @@
 
 docs:
   stage: build
-  image: python:3.7-alpine
+  image: python:3.7-slim
 
   before_script:
+    - apt-get update
+    - apt-get install -y --no-install-recommends gcc
+
     # TODO: we should consider also version locking these packages:
     #  ++ everything will work in the future
     #  -- we won't see any future improvements of the mkdocs and mkdocs-material packages