ci: add .git to the Docker container

Because commitlint needs a .git repository to lint all commits on a branch, we're adding it to the
Docker container.

re #121

.gitlab-ci.yml

@@ -45,8 +45,8 @@ lint:commits:
    image: $IMAGE_ORG/$IMAGE_NAME:$CI_COMMIT_SHA
    stage: test
    script:
-     - cd ${HOME}
-     - yarn commitlint --from=master --to=$CI_COMMIT_REF_NAME
+     - cp -r .git ${HOME}/ && cd ${HOME}
+     - yarn commitlint --from=origin/master --to=$CI_COMMIT_SHA
 
 test:
   image: $IMAGE_ORG/$IMAGE_NAME:$CI_COMMIT_SHA