Create .NET Core 2.0 docker images

[natemcmaster]

Once available, the latest tag should move to 2.0 and lts should move to 1.1.

The 2.0 images will need to react to changes in how crossgen assets are loaded:

• DOTNET_HOSTING_OPTIMIZATION_CACHE has been retired Lighting up Shared packages dotnet/core-setup#1330
• Package store layout on disk is different from current optimization cache. See https://github.com/dotnet/core-proposals/blob/master/DotNetCore-SharedPackageStore.md