You say it has worked on other images? Are you sure you've really tested that well? In general, you're doing it wrong.
In unRAID, most of the "usual" Linux locations are in RAM, will not survive a reboot, and have limited space anyway. There are really only 2 places you can store something.
1) /boot is your flash drive. This is mainly intended for saving configuration and some people use if for scripts or other small files that aren't written much.
2) /mnt is the mount point for your disks and user shares.
Please see the Docker FAQ and other stickies in the Docker Engine section.