Consistent environment variable names for DOTNET_RUNNING_IN_CONTAINER

[fbeltrao]

Steps to reproduce the issue

1. Build .NET Core application based on microsoft/dotnet:2.1-aspnetcore-runtime-nanoserver-1709
2. Build .NET Core application based on microsoft/dotnet:2.1-aspnetcore-runtime
3. Verify that nano server uses DOTNET_RUNNING_IN_CONTAINERS while the later uses DOTNET_RUNNING_IN_CONTAINER

Expected behavior

Both base image define the same environment variable name

Actual behavior

They have distinct environment variable names

Reference files

https://github.com/dotnet/dotnet-docker/blob/master/2.1/aspnetcore-runtime/nanoserver-1709/amd64/Dockerfile#L36
https://github.com/dotnet/dotnet-docker/blob/master/2.1/runtime-deps/stretch-slim/amd64/Dockerfile#L21

My proposed solution would be to change all windows based images to use DOTNET_RUNNING_IN_CONTAINER (singular). See them here

I have created PR 678 as an optional solution.

[MichaelSimons]

@fbeltrao - Thanks for logging this issue. When fixing this, I don't think we should be simply renaming the existing variable in the nanoserver images to be singular. As you noted in the PR this is considered a breaking change.

Since 2.1 is going to be LTSC, I am in favor of adding the singular version. In the 2.2 images (which are in preview) and going forward, we can make the correction so that there is only on variable.