From the OP:
"The Docker engine is running in an Alpine Linux distribution on top of an xhyve Virtual Machine on Mac OS X or on a Hyper-V VM on Windows"
This is achieved by using the native virtualisation support on OSX / Windows instead of Virtual Box and working closely with Apple and Microsoft.
From the OP:
"The Docker engine is running in an Alpine Linux distribution on top of an xhyve Virtual Machine on Mac OS X or on a Hyper-V VM on Windows"