There are two parts... there's the Linux Subsystem for Windows, which is the Linux ABI... Ubuntu for Windows is all the Ubuntu userland running via LSW. It's probable that this Docker system works with LSW as-is.
My question is why run on Windows? Wouldn't it be better for the host to be GNU/Linux if you don't need any of the functionality that Windows provides?
I fail to see the benefit of using Windows on the server unless you need Windows. Even Microsoft send to acknowledge that. I must be missing something.
They're probably doing it so that docker containers running in Azure infrastructure can be a bit lighter than it currently is. That's probably the main reason. Second, if you are already running windows servers, because you have software/infrastructure that require it, this allows you to also run Docker images, and Linux software without the overhead of full virtualization and/or new hardware.
Beyond this, there are a lot of developers running windows either by choice, or because they have software requirements themselves that can leverage this.
I fail to see the benefit of using Windows on the server unless you need Windows. Even Microsoft send to acknowledge that. I must be missing something.