Good… I found it. The host has umask 0027 set for all accounts. That caused the dirs to be created with 750, and files with 640. Since there is no uid mapping done on container level, the process that django runs as can’t open the files → No config.
Changing the perms from 750 → 755 and 640 → 644 fixed the issue. We might want to do a pre-check on this, to just abort if the config can’t be read.