This is non-standard AFAIK. Is there a reason this isn't using the normal MediaWiki qunit runner?
|integration/config : master||BABEL_CACHE_PATH in npm image for Popups|
|integration/config : master||Move mediawiki/extensions/Popups back to nodepool npm-test|
It seems like QUnit has nothing to do with the problem.
From the logs it seems like babel is trying to create a directory for its compilation cache but there is no access to the file system in the new docker based setup.
babel-register is used in development to run code with EcmaScript modules on node (for the tests).
Seems like the old setup had access to a file system and this one doesn't?