If you find me funding, I could give you CI for mobile app development, a decent MVP, in 3 months (which is me taking how long i think it takes and multiplying by 3) :P
But, like a previous poster said, it _WOULD_ be in violation of Apple's ToS. So a legal budget would be required.
We actually need something like this internally for non mobile app reasons. We build telepresence.io at Datawire and macOS dev and testing is a huge pain. Interested in a job :) ?
Heck, if it's just iOS development, it might be possible to do a cloud CI that doesn't violate the ToS. When I was building all my iOS apps from Linux, I wasn't using osx.... I was using cross compiled clang....