Authorised key path
One thing I struggled to understand while reading README is the location of authorized_keys file on Gitlab server.
My case is slightly different, namely my Gitlab is running on a virtual machine behind NAT, so no docker and volumes are involved. I figured out that the file I need is located under /var/opt/gitlab/.ssh/authorized_keys. A few things are different: dot in .ssh: not sure if its absence is a typo in README or deliberate. Secondly, my authorized_keys file wasn't empty, so I appended a key instead of overwritting the whole file with a key like it says in README.
I would improve instructions in README if you are okay with it. Otherwise, great solution! Thank you!
It seems the location of the authorized_keys has changed since the last time I tested this project. So if it works for you, then all is good.
Please be aware that the original README was written with the assumption that GitLab is running in a Docker container. I do believe that the project itself would also work with a VM.
If you have ideas to improve the README, feel free to create a pull request.