After reboot on newly installed system

# Install the 'parted' parition editor
apt-get update && apt-get install -y parted zfs-dkms zfs-initramfs

# make the big partition that is going to be the ZFS participant
range=$(parted -sm /dev/sda unit S print free | tail -n 1 | awk -F: '{print $2 " "$3}')
for d in `ls /dev/sd?`; 
do 
  parted -s $d mkpart primary $range;
done

# Create a ZFS pool named 'tank'
# Please note that I'm using the /dev/disk/by-id interface. This is more resilient than /dev/sda and /dev/sdb
zpool create -f -o ashift=12 -O atime=off -O dedup=off -O compression=lz4  tank raidz `ls /dev/disk/by-id/ata-*-part3` 

# Create OS partiton
zfs create tank/os

# Rsync the current system to the new partition.
rsync -a --one-file-system / /tank/os/

# Chroot into the system
cd /tank/os
mount --bind /dev dev
mount --bind /proc proc
mount --bind /sys sys
mount --bind /run run
chroot .

# Install GRUB into the drives
export ZPOOL_VDEV_NAME_PATH=YES
update-grub
for d in `ls /dev/sd?`; 
do 
  grub-install $d;
done

zfs set mountpoint=/ tank/os

Hetzner Ubuntu 18.04 ZFS install - rescue script

# nuke all the partition info on all the sd? disks
for d in `ls /dev/sd?`; 
do 
  for p in `parted -s $d print | awk '/^ / {print $1}'`; 
  do 
    echo "deleting $d $p";
    parted -s $d rm $p;
  done;
done

# Install Ubuntu 18.04 on a 4G partition using the Hetzner 'installimage' script
/root/.oldroot/nfs/install/installimage -a -n recoveryos -r yes -l 1 -p /:ext4:4G -K /root/.ssh/robot_user_keys -i /root/.oldroot/nfs/install/../images/Ubuntu-1804-bionic-64-minimal.tar.gz

# Reboot the system.
/sbin/shutdown -r now

Reference partition layout

root@recoveryos ~ # parted -l
Model: ATA TOSHIBA MG04ACA6 (scsi)
Disk /dev/sda: 6001GB
Sector size (logical/physical): 512B/4096B
Partition Table: gpt
Disk Flags:

Number  Start   End     Size    File system  Name     Flags
 2      1049kB  2097kB  1049kB                        bios_grub
 1      2097kB  4297MB  4295MB                        raid
 3      4297MB  6001GB  5997GB  zfs          primary


Model: ATA TOSHIBA MG04ACA6 (scsi)
Disk /dev/sdb: 6001GB
Sector size (logical/physical): 512B/4096B
Partition Table: gpt
Disk Flags:

Number  Start   End     Size    File system  Name     Flags
 2      1049kB  2097kB  1049kB                        bios_grub
 1      2097kB  4297MB  4295MB                        raid
 3      4297MB  6001GB  5997GB  zfs          primary


Model: ATA TOSHIBA MG04ACA6 (scsi)
Disk /dev/sdc: 6001GB
Sector size (logical/physical): 512B/4096B
Partition Table: gpt
Disk Flags:

Number  Start   End     Size    File system  Name     Flags
 2      1049kB  2097kB  1049kB                        bios_grub
 1      2097kB  4297MB  4295MB                        raid
 3      4297MB  6001GB  5997GB  zfs          primary


Model: ATA TOSHIBA MG04ACA6 (scsi)
Disk /dev/sdd: 6001GB
Sector size (logical/physical): 512B/4096B
Partition Table: gpt
Disk Flags:

Number  Start   End     Size    File system  Name     Flags
 2      1049kB  2097kB  1049kB                        bios_grub
 1      2097kB  4297MB  4295MB                        raid
 3      4297MB  6001GB  5997GB  zfs          primary