Support for latest dotnetcore runtimes

[sowsan]

Hello,

When I run the following CLI az web app list-runtimes --linux, I am getting the below list of Dotnet core runtimes. But this repo not covering the latest dotnet core images. Referring this post, my understanding is Linux for the web app is ultimately hosting the apps in docker containers (https://azure.microsoft.com/en-us/blog/webapp-for-containers-overview/)