Lynn, Microsoft has recently published a guide to SQL Server 2017 licensing, and in that they outline what is referenced above, that each of the official Microsoft SQL Server containers will be licensed as a VM (making it a 4 core license for each instance). Not presumably a big concern for SQL Server Dev instances, but would be potentially for Reporting/BI.
This is one advantage that Windocks independent port brings to market, which is our containers are simply named instances and are "free" under all existing SQL Server licenses. So, for those concerned about licensing costs, Windocks does offer a way to work with containers under the "named instance" provision in MS licenses.
VM's and named instances are apples and oranges. Also, If I have a four core VM and license SQL Server on that VM with four cores, I can install a default instance and multiple named instances under that on lisence. That is different from having to license multiple VM's each running a single default instance.