Merge branch 'issue-2' into 'master'

Update base images to include new dependencies

See merge request pages/mkdocs!9

.gitlab-ci.yml

-image: python:alpine
+image: python:3.6-buster
 
 before_script:
   - pip install mkdocs

README.md

@@ -15,7 +15,7 @@ This project's static Pages are built by [GitLab CI][ci], following the steps
 defined in [`.gitlab-ci.yml`](.gitlab-ci.yml):
 
 ```
-image: python:alpine
+image: python:3.8-buster
 
 before_script:
   - pip install mkdocs