Add environment variables to baser images

[richlander]

There are a set of environment variables that are useful for certain scenarios. They are either not set or set in the wrong place. I have taken a dependency on these environment variables being set in dotnet images in #420.

The variables:

• ASPNETCORE_URLS=http://+:80
• DOTNET_USE_POLLING_FILE_WATCHER=1

The ASPNETCORE_URLS variable should be set in the runtime-deps images. This enables self-contained apps and every other ASP.NET Core scenario.

The DOTNET_USE_POLLING_FILE_WATCHER variable should be set in the SDK image. This is where dotnet watch lives and it doesn't without this variable being set.

These changes should be made for .NET Core 2.1 Preview 2.

[MichaelSimons]

Thought on setting ASPNETCORE_URLS=http://+:80 in the SDK image as well?

[natemcmaster]

+1 for adding to SDK too. Although not necessarily recommended, people definitely do "dotnet run" in the SDK image to launch their aspnet apps.

[richlander]

Needed for dotnet watch run ;)

Anyway, added it.

[richlander]

This is resolved with #428