I used ls -al to display permissions of all files inside /config folder. Compared with the nearby file “apps.config.php”, config.php has no “Read” permission for “Others”, so that might be the cause of this error.
After I added “Read” permission with chmod to config.php, it still not worked well. Finally I also added “Write” permission, and it turned to be normal.
Corect - uid 501 means a non-existing user as owner. Group “dialout” also is not correct.
The permissions should be the same as in the entry above as well as owner and group.
The wrong permissions are resut of copying the config.php into the docker container, where uid 501 is different to uid 501 in the host-environment.