Merge branch 'ci-commit-ref-name-master-docker' into 'master'
Define env variables of --build-arg in Dockerfile.master
Closes #332 (closed)
See merge request gitlab-com/gitlab-docs!414
(cherry picked from commit fdd4a244)
3198ff5b Define env variables of --build-arg in Dockerfile.master f4c7c76f Catch the possibility of an empty CI_COMMIT_REF_NAME