1.) Containers are not available on desktop SKUs. So there is no licensing consideration for Windows 10. If they later add containers to Windows 10, then they'll release licensing rules at that time.
2.) If you are running a server OS for your desktop, then the licensing is pretty clear. Hyper-v VM containers follow the same rules as normal hyper-v VMs (1 physical + 2 virtual for standard then each additional VM requires a license, unlimited for datacenter). For Windows Server Containers (which are not hyper-v based), it's even easier, it's unlimited regardless of edition.
3.) As always, the host OS must be licensed fully in order to have the appropriate rights (2016 is moving to core licensing, with a minimum of 8 core licenses per processor, and a minimum of 2 processors).
All in all, it's one of the easier features to understand the licensing for since it doesn't directly deal with CALs or internal/external usage rights.