Enable environment variable expansion

[f1-outsourcing]

now this is the command:
/usr/sbin/iptables -t filter -A OUTPUT -o $CNI_IFNAME -p icmp -d 0/0 --icmp-type 8 -j ACCEPT --wait

CNI_IFNAME is listed as interface in the iptables output

[@ firewallnetns]#ip netns exec testing iptables -L -vn
Chain INPUT (policy ACCEPT 0 packets, 0 bytes)
 pkts bytes target     prot opt in     out     source               destination
    0     0 ACCEPT     icmp --  $CNI_IFNAME *       0.0.0.0/0            0.0.0.0/0            icmptype 8
    0     0 ACCEPT     icmp --  $CNI_IFNAME *       0.0.0.0/0            0.0.0.0/0            icmptype 0
    0     0 ACCEPT     all  --  *      *       192.168.122.159      0.0.0.0/0

Chain FORWARD (policy ACCEPT 0 packets, 0 bytes)
 pkts bytes target     prot opt in     out     source               destination

Chain OUTPUT (policy ACCEPT 0 packets, 0 bytes)
 pkts bytes target     prot opt in     out     source               destination
    0     0 ACCEPT     icmp --  *      $CNI_IFNAME  0.0.0.0/0            0.0.0.0/0            icmptype 0
    0     0 ACCEPT     icmp --  *      $CNI_IFNAME  0.0.0.0/0            0.0.0.0/0            icmptype 8

should be this:
/usr/sbin/iptables -t filter -A OUTPUT -o eth0 -p icmp -d 0/0 --icmp-type 8 -j ACCEPT --wait