Cross-CI on Windows and Mac
There are some fundamental misunderstandings in your bullets which makes your question unclear:


Apple will not allow you to develop/build using a virtual system.  I believe this is a legal constraint.  That said, you can run pretty much anything in VirtualBox as long as you have a legally provided image (e.g. a Linux image).
You can only run the Windows base images if you are running Windows Containers which will be incompatible with MacOS systems.  If you want to build on MacOS using Docker you will need to use Linux containers.


However, you can run your continuous-integration runners across Windows and Mac.  This is one of the major benefits of implementing runners in Docker for your CI tool.  Now if you have agents on Windows, MacOS, or even Linux, you can get a consistent build experience by using Docker. This official GitLab article has more about setting this up.