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.
Are you an iOS developer? Because you are repeatedly taking what is considered run of the mill by iOS developers as if it were some outlandish concept you’ve never heard of before. Build servers – yes, with free tiers – have been commonplace for iOS apps for at least a decade.
And you still can't share that app.