Update .gitlab-ci.yml

.gitlab-ci.yml

-image: python:3.6-alpine
+image: python:3.6
 
+cache:
+  paths:
+    - .pip
+
+before_script:
+  - mkdir -p .pip
+  - pip install -U pip
+  - pip --cache-dir=.pip install sphinx
+  - pip --cache-dir=.pip install sphinx-autodoc-typehints
+  - pip --cache-dir=.pip install sphinx_rtd_theme
+  - pip --cache-dir=.pip install -r requirements.txt
+  
 pages:
   script:
-  - apk --no-cache add py3-pip python-dev
-  - pip install sphinx
-  - pip install sphinx-autodoc-typehints
-  - pip install sphinx_rtd_theme
-  - pip install -r requirements.txt
   - apk --no-cache add make
   - cd docs && make html
   - mv build/html/ public/