fix(contrib/gce): fix docker storage being wiped on boot

Ref: #1868

Author: Xe

contrib/gce/gce-user-data-template

@@ -6,11 +6,13 @@ coreos:
       content: |
         [Unit]
         Description=Formats the ephemeral drive
+        ConditionPathExists=!/etc/gce-formatted
         [Service]
         Type=oneshot
         RemainAfterExit=yes
         ExecStart=/usr/sbin/wipefs -f /dev/disk/by-id/scsi-0Google_PersistentDisk_coredocker
         ExecStart=/usr/sbin/mkfs.btrfs -f /dev/disk/by-id/scsi-0Google_PersistentDisk_coredocker
+        ExecStart=/bin/touch /etc/gce-formatted
     - name: var-lib-docker.mount
       command: start
       content: |