Add QT_QPA_PLATFORM=offscreen by default

Thanks to @jkende setting the env var to QT_QPA_PLATFORM allows lib using qt to work in the docker image.

I think we can add it to our default image.

I propose we distinguish env var at the begining we need from the env var at the end that may be needed by end users

"might" need !19 (merged) to work...

requested review from @echapeau

added 3 commits

• 134ee8b7...d4066759 - 2 commits from branch main
• ff7bff29 - Merge branch 'main' into qt-patch

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mentioned in commit cb088697

merged