In your case the -i <src_if> src_if would be your lxdbr0 interface and -o dst_if would be your external interface.
However you could relax this further and just use:
iptables -I DOCKER-USER -j ACCEPT
To allow all traffic.
However the reason there isn’t a canonical answer for this is that it all depends on your particular networking setup and your security stance (allowing all traffic maybe not what you want for example).