Add iptables allow rules for mgmt bridge in the ingress direction

[hellt]

After #2397 and #2401 we add the following rule (v4 example):

❯ sudo iptables -vnL DOCKER-USER                                   
Chain DOCKER-USER (1 references)
 pkts bytes target     prot opt in     out     source               destination         
 5021  384K ACCEPT     0    --  *      br-6fd1260303e1  0.0.0.0/0            0.0.0.0/0            /* set by containerlab */
  878 1329K RETURN     0    --  *      *       0.0.0.0/0            0.0.0.0/0           

since the mgmt bridge is only specified in the outgoing direction, the traffic from another docker network targeting clab network will still be blocked; we need an allow rule also in the in direction

[hellt]

done in #2406