When booting your images, you will most likely want to mount some partitions to persist your data. For example, a common one would be:
/dev/sda2 /home ext4 defaults 0 0
This would mean your entire operating system will be cleaned after every reboot, except everything under /home.
To configure an /etc/fstab to be used during boot, create a /etc/darch/hooks/fstab.confg file.
image:name=this_fstab_file
*=that_fstab_file
Then place your fstab in /etc/darch/hooks. For example, /etc/darch/hooks/this_fstab_file.