ASP .NET 5 uses DNX and the standard build in Visual Studio Code does not work.
You have to create your on build as describe in this article:
But in the tutorial there is missing one important step:
In the Visual Studio Build Step you have to uncheck "Restore NuGet Packages"
Without this you will get
##[error]Error parsing solution file at C:\a\1\s\src\simashr\simashr.xproj: Exception has been thrown by the target of an invocation.
##[error]Unexpected exit code 1 returned from tool NuGet.exe