Docker is based heavily on libcontainer (their replacement for LXC), which is based very heavily on kernel namespaces and cgroups. Those features are Linux-specific, and are a very important and fundamental part of what lets Docker containers work. Somebody could totally write another container backend to replace libcontainer that supported the mach kernel or other BSDs or Hurd or whatever, but it would likely require a lot of work (ongoing work, considering the speed at which Docker is changing over time).
That's what I'm saying. I'm really appalled at the amount of downvotes that I'm getting for having the guts to say "Windows" in a Docker thread, but with a gigantic effort, it's totally imaginable to have a Docker-like system on other kernels than Linux. People who can't imagine that are just, well, unimaginative.
