So, to clarify things: it's QEMU running in a container, and macOS running under QEMU inside it.
That means, Docker isn’t really running MacOS.
This is really nice WRT the ease of installation: no manual setup steps and all.
This likely expressly violates the macOS EULA], which says: «you are granted a limited, non-exclusive license to install, use and run one (1) copy of the Apple Software on a single Apple-branded computer at any one time» — because the point is to run it not on a Mac. So, pull it and keep it around; expect a CD letter come any moment.
Exclamation mark!