[nightly] Build images for 2.1.0-preview1

[natemcmaster]

Fixes #279

Changes:

microsoft/aspnetcore-nightly

• No longer install the ASP.NET Core package runtime store. This has been replaced by the ASP.NET Core shared runtime
• Base image is now microsoft/dotnet-nightly:2.0-runtime-deps instead of 2.0-runtime. ASP.NET Core installers include the Microsoft.NETCore.App shared runtime

microsoft/aspnetcore-build-nightly

• Remove bower and gulp
• Install NodeJS 8.9.3 - #334 -Install Git 2.16.1 (NanoServer only) Removed git from the images.
• Set WORKDIR /root/code. Helps with the common pitfall of trying to build the app in /, which fails b/c MSBuild tries to overwrite /bin.
• Update base image to microsoft/dotnet-nightly:2.1.300-preview1-sdk
• Don't run dotnet-restore when building the container. All packages should already exist in the NuGet fallback folder

Note: CI is expected to fail until microsoft/dotnet-nightly:2.1.300-preview1-sdk images are available.

cc @MichaelSimons @glennc

[MichaelSimons]

Required microsoft/dotnet-nightly changes are in https://github.com/dotnet/dotnet-docker-nightly/pull/546

[natemcmaster]

@MichaelSimons updated with cleanup according to your feedback.