Add support for dotnet-isolated for command func init --docker

For dotnet, following instructions here:

https://docs.microsoft.com/en-us/azure/azure-functions/functions-create-function-linux-custom-image?tabs=bash%2Cportal&pivots=programming-language-csharp#create-and-test-the-local-functions-project

generated docker file with dotnet base image
```
# To enable ssh & remote debugging on app service change the base image to the one below
# FROM mcr.microsoft.com/azure-functions/dotnet:3.0-appservice
FROM mcr.microsoft.com/azure-functions/dotnet:3.0
```

Need similar support for dotnet-isolated that uses base image that includes dotnet 5