If I rent a dedicated machine I can certainly do what I described there[0] by hand but would you envision having some kind of turn-key support for GitLab CI runners, supporting both gitlab.com through some partnership and self-hosted instances? That would be brilliant, especially considering Xcode integration with GitLab.
[0]: https://news.ycombinator.com/item?id=18404893
[1]: https://about.gitlab.com/2018/06/06/one-click-clone-to-xcode...