We had updated to a very new version from bullseye-backports to test
if it had a necessary bugfix, but that brought no improvement. As it
turned out, the problem was ulimit -n as addressed in 03f46509.

2.2.9 is sufficient for our purposes. backports repos are problematic
because a version of a package ceases to exist from time to time.
This causes container builds to fail at times that are unpredictable
in practice.