Store the CI caches inside the project dir since that's the only place GitLab CI is allowed to cache

.gitlab-ci.yml

@@ -1,11 +1,13 @@
 image: python:3.6
 variables:
   GIT_SUBMODULE_STRATEGY: normal
+  PIP_CACHE_DIR: $CI_PROJECT_DIR/.pip-cache
+  PIPENV_VENV_IN_PROJECT: true
 
 cache:
   paths:
-    - ~/.cache/pip
-    - ~/.local/share/virtualenvs
+    - .pip-cache
+    - .venv
 
 test:
   script: