Add .NET Framework 3.5 Builder Image

[richlander]

We recently added builder images. These images are specific to .NET Framework 4.x and have no allowance for building .NET Framework 2.0 and 3.x applications. That is obvious in microsoft/dotnet-framework-docker-samples#16.

We should create 3.5 images. I'm thinking that they should be based on the 3.5 image, add the 4.latest runtime and then otherwise look identical to the 4.x builder images.