add: iptables in DinD image

[rojomisin]

What

adds iptables package to the k3d image

Why

When attempting to run on OS X Docker 20.x, I was getting the error:

docker run --privileged --name=k3d --hostname=k3d -p 6443:6443  rancher/k3d 
...
subject=CN = docker:dind client
Getting CA Private Key
/certs/client/cert.pem: OK
ip: can't find device 'ip_tables'
modprobe: can't change directory to '/lib/modules': No such file or directory

...
level=warning msg="Running iptables --wait -t nat -L -n failed with message: `iptables v1.8.7 (legacy): can't initialize iptables table `nat': iptables who? (do you need to insmod?)\nPerhaps iptables or your kernel needs to be upgraded.`, error: exit status 3"

Implications

seems like a safe fix on my custom built image

[iwilltry42]

Didn't see that needed before, but won't harm. The DinD image is for testing/dev mostly, so shouldn't be critical.
Thanks @rojomisin !