unable to create files from linux library via php
I finally was able to find a solution. In a separate container, I had a process supervisor running, who saw this file, because it was mounted to the main application directory also. What had to be done was to mount the file to the main container and the supervisor container.
Try
exec("traefik-certs-dumper file --source acme.json --dest some_dir --version v2");intead of system()