pass CONAN_USERNAME envvar to docker (#479)

[Artalus]

fixes #479

Changelog: Fix: Fix empty username on Docker

Due to _noneize() being used during construction of ConanFileReference, the user and channel parts of reference object are replaced from '_' to None.
Thus, when user provides CONAN_USERNAME=_, CONAN_CHANNEL=_ for CPT to be forwarded to docker run, the username var was reset to None, causing Conan running inside docker to think that the user provided only one envvar.

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[kasunch]

I have been using this fix in my fork for sometime with the hope that this will be merged eventually.
Is there any reason why this hasn't been merged yet?

[uilianries]

It sounds good for packages which doesn't contain user/channel. I gonna check a test for it first. Without a test we can't accept it.

[uilianries]

I've opened PR #522 over this PR, it contains an integration test which is possible to validate your hotfix. That PR respects your commit (author) and will close this one as soon as it gets merged.