You can read more about Docker for Windows here: https://blog.docker.com/2016/03/docker-for-mac-windows-beta/
When building Docker for Windows, we wanted to build an integrated and native-feeling product. Having VirtualBox as a separate install with Toolbox has proved troublesome and error-prone.
This came at the cost of not working with VirtualBox. If you use VirtualBox, you can use Docker Toolbox - it’s continuously updated with the latest Docker Engine releases.