fix mkdocs-material

https://github.com/squidfunk/mkdocs-material/commit/5b1dcacd37f62cf79cd7ee8deefff22050697535

Dockerfile

@@ -7,7 +7,7 @@ WORKDIR /app
 # add the current directory to the container as /app
 ADD . /app
 # Perform build and cleanup artifacts
-RUN pip install mkdocs && pip install mkdocs-material && pip install plantuml-markdown
+RUN apk add --no-cache --virtual .build gcc musl-dev && pip install mkdocs && pip install mkdocs-material && pip install plantuml-markdown && apk del .build gcc musl-dev
 
 # build the documentation site
 RUN mkdocs build