The first subnet of each docker bridge networks with their routes. It is supposed the sought IPv4 subnet is the first one (with index 0 in Config
map)
for n in $(docker network ls --format={{.ID}} --filter='driver=bridge'); do; printf "%s %-30s" $n $(docker inspect $n --format='{{.Name}}'); ip route | grep $(docker inspect $n --format='{{$value := index .IPAM.Config 0}}{{$value.Gateway}}'); done