With Asp.Net Core we can build the platform independent application and with containerization approach too the same old existing IIS dependent application can be hosted and run in different OS.
Does it mean in future if containerization approach is practiced then it won't matter in what language you are building code ?
It's the language that enables the platform independence, not the containerisation.
.NET Core was designed to be cross platform. It is able to run on Linux, Windows and macOS. This means it can also run in a container those platforms. Similar to Java, Node.js, Python etc, there are runtimes for multiple platforms and the runtimes go someway to hiding the platform details from your implementation when possible.
Microsoft is working towards a form of platform independence with it's Windows Subsystem for Linux but I'm not sure that will be used for much more than development. Linux Containers On Windows may also help blur the lines, but underneath there is a Linux VM running the Linux containers.