How can I test native macOS UI software in a container before deployment?
I can confirm that running Docker on Mac works just fine, but according to this ServerFault question, it's not possible to just run MacOS in Docker. 

(You might be able to run MacOS in a VM in a Docker container, but that's probably not what you want.)

You could purchase a MacMini, use it as a Jenkins slave and run automated tests on it as part of your build pipeline or something like that?