Perhaps technically possible, but in practice it's against apple development directives. I doubt you do this yourself, or know anyone that actually develops iOS apps from a non-mac in the real world.
MacOS TOS clearly state that if you're someone renting out access to macOS, you may only rent it for periods longer than 24h. I heavily doubt bitrise does that, as cost would be prohibitive (oh hey I wonder why that is)
Oh and up until 2019 apple was forbidding any virtualization of macOS in its EULA, there certainly were no free cloud build servers for iOS since relatively recently.