Windows 10 will soon ship with a full, open source, GPLed Linux kernel
The new Windows Subsystem for Linux will use a real Linux kernel.
arstechnica.com
The current Windows Subsystem for Linux uses a Microsoft-authored kernel component that provided the same kernel API as the Linux kernel but written from scratch by Microsoft. Essentially, it translated from Linux APIs to Windows NT kernel APIs. That worked pretty well, but the current subsystem had a few shortcomings: there was no ability to use Linux drivers, in particular file system drivers.
All is changing with Windows Subsystem for Linux 2. Instead of emulating the Linux kernel APIs on the NT kernel, WSL 2 is going to run a full Linux kernel in a lightweight virtual machine. This kernel will be trimmed down and tailored to this particular use case, with stripped-down hardware support (since it will defer to the host Windows OS for that) and faster booting.
By using the Linux kernel itself, Microsoft gets all of Linux's features for free. This is why WSL 2 will support Docker containers: all the underlying infrastructure, such as cgroups, is already in the Linux kernel, and Microsoft won't need to implement the features itself. The embedded kernel will be serviced and updated by Windows Update.