Update docker.service

Docker startup fails
You specified --iptables=false with --ipmasq=true. IP masquerading uses iptables to function. 
Please set --ipmasq to false or --iptables to true

docs/getting-started-guides/coreos/units/docker.service

@@ -5,7 +5,7 @@ Documentation=http://docs.docker.io
 
 [Service]
 ExecStartPre=/bin/mount --make-rprivate /
-ExecStart=/usr/bin/docker -d -s=btrfs -H fd:// -b cbr0 --iptables=false
+ExecStart=/usr/bin/docker -d -s=btrfs -H fd:// -b cbr0 --iptables=false --ip-masq=false
 
 [Install]
 WantedBy=multi-user.target