After installing coreos on a hetzner server i realized that fleet unit files pulling from docker fail on boot, because there was no dhcp lease yet.
dial tcp: lookup index.docker.io: connection refused
journalctl shows that dhcp gets an ip after starting the container unit file, therefore docker pull fails.
First you have to add a network dependency to your unit file:
Wants=docker.service etcd.service network-online.target
After=docker.service etcd.service network-online.target
But the network-online.target does not work unless you enable another service first:
systemctl enable systemd-networkd-wait-online.service
This sadly does not work on current CoreOS stable 522.5.0 but on beta 557.0.0