I think there are two questions in this thread.
1) how do containers present an opportunity to save on license costs (the original question)?
The answer for users of Microsoft's SQL containers is perhaps there is no savings opportunity, as each container will be licensed as if a VM. For users of Windocks SQL Server containers, however, a single 4 or 8 core license can support scores of containers under existing SQL licenses. We have customers who run 30 or more containers on a single 8 core server. This is doable, as Windocks SQL containers are supported without additional licensing, as named instances.
2) the second question is why use containers when the functional equivalent would be named instances on a VM?
Docker containers deliver flexibility to create and replace environments, where conventional named instance configuration is fairly set and requires scripts to update/change. Containers can be tailored to support specific configurations, and delivered and replaced within a minute (including database mail, encryption, etc.). Containers also enable use of secure Docker images that can include complex data environments that can include scores of databases. Final point would be to ask why Microsoft is placing such emphasis on use of containers for sQL Server 2017? The reason is superior support for automation, DevOps, and Continuous Integration.
Hope that helps, Paul