Drone-runner-kube with parallel docker builds

Whenever we try to build 2 containers at once; by using parallel steps; we frequently
get the error:

Cannot connect to the Docker daemon at unix:///var/run/docker.sock. Is the docker daemon running?re

If we make the steps sequential, it works. From what I can tell; it starts a pod with a container for
each step; each one tries to launch the daemon; I enabled debugging and I saw the following errors:

failed to start daemon: Error initializing network controller: error obtaining controller instance: failed to create FILTER isolation chain: iptables failed: iptables --wait -t filter -N DOCKER-ISOLATION-STAGE-1: iptables: Chain already exists.
 (exit status 1)


Please see the following threads that discuss limitations around running multiple docker-in-docker containers in the same Pod, as well as potential workarounds:

This is so needed; why is this dead? add iptables option to the daemon by sherry-ummen · Pull Request #309 · drone-plugins/drone-docker · GitHub

The author has not addressed pull request review comments, which could otherwise cause a regression. If pull request comments are addressed, we can merge.