How does licensing work?
For production, licensing is at the host level, i.e. each machine or VM which is running Docker. Your Windows licence on the host allows you to run any number of Windows Docker containers on that host. With Windows Server 2016 you get the commercially supported version of Docker included in the licence costs, with support from Microsoft and Docker, Inc.
For development, Docker for Windows runs on Windows 10 and is free, open-source software. Docker for Windows can also run a Linux VM on your machine, so you can use both Linux and Windows containers in development. Like the server version, your Windows 10 licence allows you to run any number of Windows Docker containers.
Windows admins will want a unified platform for managing images and containers. That’s Docker Datacenter which is separately licensed, and will be available for Windows soon.