In all cases, the Docker daemon is running under Linux. The Mac and Windows versions are merely bundling up a Linux VM containing Docker with a frontend that's as transparent as possible, but still with Linux as a hard requirement.

Pretending that the Mac and Windows versions somehow aren't using Linux VMs behind the scenes is of no use to anyone. It's a convenience for users when they can get by with ignoring the VM layer, but a detriment when we see people start talking as though Docker for Mac is functionally different from a Linux VM running Docker, and start assuming that enhancements to running Linux VMs under macOS would be inapplicable to and incompatible with "Docker for Mac".

> In all cases, the Docker daemon is running under Linux. The Mac and Windows versions are merely bundling up a Linux VM containing Docker with a frontend that's as transparent as possible, but still with Linux as a hard requirement.

I really doubt that's the case if you run native Windows containers on Windows.

Thanks for pointing that out. I hadn't realized Microsoft had jumped on the Docker bandwagon to that extent; it's far enough from the topic at hand and from anything I'd ever use that I overlooked it.

So while there is in fact an exception to my previous generalization, there's still no cross-platform compatibility magic to Docker aside from that of virtual machines. If the container OS is different from the host OS (or a different version of the OS, for Windows containers), then using Docker is an instance of using VMs, not an alternative to VMs.