We found the reason why the networking is not working.
IP forwarding needs to be enabled manually in the docker ingress-sbox namespace:
nsenter --net=/run/docker/netns/ingress_sbox sysctl -w net.ipv4.ip_forward=1
Why is that required in an LXD container and not in a “normal” environment? Any help is very much appreciated.
I also posted this issue in the docker forum as it might require some expertise on both areas.