Where can I config service startup options in Ubuntu?

Ubuntu and Debian use Upstart as a replacement for init, but legacy init scripts are supported. Chances are that you can add the option in a script in /etc/init.d with the same name as the service. Also look in /etc/init for .conf files that control the runlevels that services are started and stopped on and how they're respawned, etc.

First find your config you want to change using the following command:

systemctl list-unit-files

Then change the specific config using the following command:

sudo systemctl edit --full rocketchat.service