(Replaced by #680) Consistent environment variable names for DOTNET_RUNNING_IN_CONTAINER

[fbeltrao]

This PR is a potential fix for issue #677

I am however, not sure about the side effects of this change. It will potentially break any existing application that expects the old behavior. We could provide both for backward compatibility giving time for people to make code changes.

Another option is to fix this problem only in newer versions.

[MichaelSimons]

@fbeltrao - Thanks for making this PR. As I noted in the #677 - I'm of the opinion that we should not make this a breaking change. My preference would be to add the singular variable in the 2.1 images and then fix the problem going forward. Lastly can you make this change in the nightly branch? Nightly includes 2.2 and 3.0 images as well. The changes will be available immediately in nightly and then get merged into master during the next patch Tuesday.

[fbeltrao]

@MichaelSimons thanks for the feedback. Just to make sure I understand your suggestions.

• To fix 2.2+ versions (currently in preview) we can make the breaking change, having only DOTNET_RUNNING_IN_CONTAINER for all images. This PR should be made on nightly branch.

• To fix 2.1 version (with LTSC) we cannot introduce a breaking change, therefore we should define both values (singular and plural) on windows images. The PR for this change should go to $master branch.

[MichaelSimons]

@fbeltrao - One small correction - both changes should go into nightly. The 2.1 changes will get merged into master during the next .NET servicing event (e.g. patch Tuesday).

[fbeltrao]

@MichaelSimons new PR made on nightly branch: #680