community/docker: add ip6tables dependency to docker-engine
docker-engine already depends on iptables, but also requires ip6tables:
WARN[0000] Failed to find ip6tables: exec: "ip6tables": executable file not found in $PATH
docker-engine already depends on iptables, but also requires ip6tables:
WARN[0000] Failed to find ip6tables: exec: "ip6tables": executable file not found in $PATH