Windows containers don't have the same licensing requirements as servers or VMs running Windows. Windows is licensed at the host level, not the container level. If you have 100 Windows containers running on one server, you only need one license for the server. There are considerable savings to be had if you currently use VMs to isolate application workloads. Removing the VM layer and running apps in containers directly on the server removes the licensing requirement for all of the VMs, and the management overhead for all those machines.

Hyper-V containers have separate licensing. On Windows 10 you can run multiple containers, but not for production deployments. On Windows Server, you can also run containers in Hyper-V mode to increase isolation. This can be useful in multi-tenant scenarios, where you need to expect and mitigate for hostile workloads. Hyper-V containers are separately licensed, and in a high-volume environment, you need a Windows Server Datacenter license to run Hyper-V containers without individual licenses.

Microsoft and Docker, Inc. have partnered to provide Docker Enterprise at no cost with Windows Server 2016 and Windows Server 2019. The price of the Windows Server license includes the Docker Enterprise Engine, which gives you support to run applications in containers. If you have problems with a container or with the Docker service you can raise it with Microsoft, and they can go on to escalate it to Docker's engineers.