lots of same messages flood into docker logs

updown: Perhaps iptables or your kernel needs to be upgraded.
updown: iptables v1.6.0: can't initialize iptables table `filter': Permission denied (you must be root)

ENV: Ubuntu 16.04.2 LTS + docker@latest

Start by reproducing the repeated updown messages in Docker logs on Ubuntu 16.04.2 with docker@latest, then inspect the reported iptables filter-table permission error and determine its relationship to the log flood. Done means the cause is identified and the repeated messages no longer flood the logs.

Written by the indexing model from the issue text.