Refactor common logic out of ImageBuilder into a shared library.

There is becoming a fair amount of logic defined within the ImageBuilder tool that could be shared elsewhere. Examples include: [FilePusher](https://github.com/dotnet/docker-tools/tree/master/eng/file-pusher), Docker repo unit tests (DockerHelper).

This shared logic could easily be factored into a dll that is easily shared within this repo. It could be shared with dependent repos via a NuGet package.