If you want to enable syslog-ng, you have to run a "sidecar" container (see syslog-ng service in docker-compose.yml example) or run a simple container like this :
docker run -d --name librenms_syslog \
--env-file $(pwd)/librenms.env \
-e SIDECAR_SYSLOGNG=1 \
-p 514 -p 514/udp \
-v librenms:/data \
librenms/librenms:latest
You have to create a configuration file to enable syslog in LibreNMS too. Create a file called for example /data/config/syslog.php
with this content :
<?php
$config['enable_syslog'] = 1;