CoreOS drive format systemd unit

# /etc/systemd/system/format-var-lib-docker.service
[Unit]
Description=Format ephemeral drive for /var/lib/docker

Before=docker.service
Before=var-lib-docker.mount
ConditionPathExists=/dev/sdb1

[Service]
Type=oneshot
ExecStart=/usr/sbin/wipefs -f /dev/sdb1
ExecStart=/usr/sbin/mkfs.ext4 /dev/sdb1