docker on Synology NAS: nftables error because ip6table_nat missing

[ymarkus]

Describe the problem
Sorry for the many issues, I've switched all my machines from wireguard to netbird and am finding some issues. 🙈
If I try to run the docker container on a Synology NAS, host routes are not set correctly and the logs show this error:

2023-05-31T09:34:56Z ERRO client/internal/engine.go:612: failed to update routes, err: couldn't initialize firewall containers, got err: nftables: unable to list tables: netlink receive: invalid argument

I'm not sure why the nftables error occurs (maybe too old kernel?). I've tried to build an alpine docker container with iptables-legacy and that shows a different error, because the kernel in the NAS is missing the ip6table_nat module.
If I set a route manually by running ip route add 100.82.0.0/16 via $DOCKER_IP it works, but the network routes are missing. Right now I just add them manually with a shell script, but that is tedious.

To Reproduce
Steps to reproduce the behavior:

1. Run docker container on Synology DSM with NET_ADMIN cap
2. Container connects, but host is missing routes

Expected behavior
Routes on the host should be set.

NetBird status -d output:
This is weird for some reason:

Peers detail:                                                                   
Daemon version:                                                                 
CLI version: 0.20.5                                                             
Management: Disconnected                                                        
Signal: Disconnected                                                            
FQDN:                                                                           
NetBird IP: N/A                                                                 
Interface type: N/A                                                             
Peers count: 0/0 Connected