It will add a Linux image in Dockerfile when adding Docker files (Windows OS selected) for a .NET project

[v-ruizh]

OS: Win 10
Build Version: 20210615.1

Repro Steps:

1. Switch the Docker Desktop to windows container.
2. Create and open a .NET Core MVC project in VS Code.
3. Add Docker files to this project with Windows OS selected.
4. Check the image name in Dockerfile.

Expect:
The image name is "xxx/aspnet:5.0".

Actual:
The image name is "xxx/aspnet:5.0-focal" which is a Linux image.

More Info:

1. Fail to debug the project with the default added image in windows container.
2. It seems a regression from this PR Use Ubuntu 20.04-based images for .NET scaffolding #2996.

[karolz-ms]

@v-ruizh correct, we switched to Ubuntu-based images because the default image does not contain binaries (ps) necessary for debugger attach (process picker) to work.

We should discuss what to do here in 1.15 timeframe