In addition to sibling suggestion of Xcode on the cloud (which is the cheapest option to start with), you can also buy a refurbished/second-hand Mac, which goes

In addition to sibling suggestion of Xcode on the cloud (which is the cheapest option to start with), you can also buy a refurbished/second-hand Mac, which goes for less than $1000, and you can definitely find some around $500. You don't even need a powerful Mac to use as a build server: I got great use of a Late 2014 Mac mini (be careful to buy a somewhat recent Mac though, to avoid being locked out of next MacOS versions) as a build server. Full builds takes up to 2 hours, but who cares? It's running nightly.