Syncthing requires a host network mode

[human5810]

Per the documentation,

Please note that Docker's default network mode prevents local IP addresses from being discovered, as Syncthing can only see the internal IP address of the container on the 172.17.0.0/16 subnet. This would likely break the ability for nodes to establish LAN connections properly, resulting in poor transfer rates unless local device addresses are configured manually.
It is therefore strongly recommended to stick to the host network mode, as shown above.

https://github.com/syncthing/syncthing/blob/main/README-Docker.md#discovery