By default, all the ports that Synergy uses that are not public are firewalled by the Docker Swarm configuration, but if you still want to use another firewall layer, here is a list of ports that must be open for full functionality:
53 (UDP+TCP)
67-69 (UDP+TCP)
80 (TCP)
443 (TCP)
88 (TCP)
139 (TCP)
445 (TCP)
873 (TCP)
3000 (TCP)
4000-4002 (TCP)