Microsoft has announced that Windows Server 2016 will be generally available sometime in October. It also announced that the Commercially Supported Docker Engine will be included with the OS for free.
In a blog post, Docker stated:
These fruits of this partnership for the first time offer enterprise IT professionals with a single platform for both Windows and Linux applications on any infrastructure, whether bare metal, virtualized, or cloud.
Microsoft has offered preview versions of Windows Server 2016 for the past several months. IT pros can download and evaluate the OS for 180 days from Microsoft's Technet site. In addition to Windows Server 2016, Microsoft also plans to make System Center 2016 generally available in October as well.