I think other than opensource dogmatism about not depending on GAFAMS (here Microsoft) and Gitlab being hosted on Azure as well, I think the main reason is we've been bitten in the past by such services, suddenly adding requirements like repo size and file size (Recently source forge asked us to delete some old releases to save space for them) Phabricator was maintained by Facebook and used by Blender until it wasn't.
That's why Gitea has been chosen, and is in good way with @Itms's work.
Moreover we always provided mirrors for those that prefer to use those services.
The lobby is on GitHub, and it has attracted nobody more than it did when using the old SVN workflow.
The community mod is hosted on Gitlab, @real_tabasco_sauce can probably tell you if they feel it's better or not.
Well, it's as simple as that, the dockerfiles are there if you're on linux, and you just have a simple shell script to run.
The main problem we had was cloning 10GB, so we mount volumes instead to reduce the load on the server.
Installing wxwidgets on Windows https://code.wildfiregames.com/P196
Building on Windows https://code.wildfiregames.com/P195
Regarding macos, it's always been an issue, they break stuff on a monthly basis (New xcode version, boom), and it's still a miracle that the team managed to keep making releases for it.
We always had to keep the CI on the lowest macos we support here it's 10.12, which has caused issues, and I bought a mac to tackle the rest, but now that I'm retired, I don't know who will keep on the patches coming.
EDIT: Phab's down again, but I think I also had a script there for macOS, assuming of course no Xcode update broke it.
EDIT2: Github seems to be moving on with this copilot thingy, and whether or not we want to train it more than we already did (Chat GPT 3 has the entire 0 A.D. codebase inside it) is also a concern.