Provide guidance on utilizing NuGet package caches for build scenarios

Within the .NET community there are users asking/discussing the best/recommended patterns for utilizing a NuGet package cache while building their .NET projects with Docker. Here is a recent [example](https://stackoverflow.com/questions/60799918/using-nuget-cache-inside-a-docker-build-with-net-core-when-offline).

There are two main scenarios which have very different solutions.

1. Building .NET projects within Dockerfiles
1. Building .NET projects within a running container

It would be beneficial to have some guidance for these scenarios. Creating a [snippet](https://github.com/dotnet/dotnet-docker/tree/master/samples/snippets) on this topic feels like a good way to capture the guidance.