dup(0) fails on Ubuntu 20.04 and macOS 10.15 with 13.0

Dear All,

I've used PostgreSQL since version 9.x successfully on Linux, macOS
and Windows. Today I've upgraded from 12.3 to 13.0 and suddenly I can
not start the server any more on Ubuntu 20.04 (inside Docker on Ubuntu
18.04) and on macOS 10.15.

I get reproducibly the error:
2020-10-05 11:48:19.720 CEST [84731] WARNING: dup(0) failed after 0 successes: Bad file descriptor
2020-10-05 11:48:19.720 CEST [84731] FATAL: insufficient file descriptors available to start server process
2020-10-05 11:48:19.720 CEST [84731] DETAIL: System allows 0, we need at least 58.
2020-10-05 11:48:19.720 CEST [84731] LOG: database system is shut down

What makes this quite curious is that everything continues to work on
Ubuntu 18.04, and Windows with Visual Studio 2019.

I compile postgreSQL myself from source, but there are no patches or
tweaks involved (that I could think relevant for this problem).

I've searched for the particular error and understand that it is usually
caused by system limits on new files(?) But in my case, the failing
setups are relatively modern, powerful CI test machines, with virtually
no load at the time of test.

On Linux Ubuntu 20.04, `ulimit -n` shows `1048576` which seems also
relatively high (but must be the default, not changed by me).

Does this problem mean anything to anyone? I'm completely lost where
to go from here, or what to try next :-( Help would be greatly
appreciated!

All the best,

Mario Emmenlauer