Cloud-builders: Add support for dotnet core 2.1 build

[Andrioden]

ASP.Net Core 2.1 was just officialy released. After i upgraded my ASP.Net project to 2.1, i am not longer able to use your cloud-build tool dotnet to automatically run my tests. It seems to be confirmed by your documentation that it is not yet supported.

gcr.io/cloud-builders/dotnet test [my-dotnetcore2.1-project]

gives

/usr/share/dotnet/sdk/2.1.4/Sdks/Microsoft.NET.Sdk/build/Microsoft.NET.TargetFrameworkInference.targets(135,5): error : The current .NET SDK does not support targeting .NET Core 2.1. Either target .NET Core 2.0 or lower, or use a version of the .NET SDK that supports .NET Core 2.1. [/workspace/[MyProject].Tests/[MyProject].Tests.csproj]

[bendory]

@ivannaranjo PTAL

[ILMTitan]

As Ivan is no longer part of cloud#, I will take a look at this.