I recently spent way too much time building a working CI pipeline for Gitlab that would upload and active my Ghost theme after every update. I kept being led to this forum without a concrete solution and vowed to share what I created if I ever got it to work. I did it, and here it is: Classmate Team / Gitlab-CI for Ghost Themes · GitLab
There’s a Javascript version, and a Bash version. You only need one, so pick whatever you’re most comfortable with. Good luck to anyone that ventures down the same path — may you find this helpful. 