Delay dockerd boot on Raspberry PI with images in external storage

[enriquevagu]

Docker fails after reboot because of a race condition between HDD spin-up and dockerd service boot. Adding a simple sleep command solves the issue.

See: https://forum.openmediavault.org/index.php?thread/30305-is-your-docker-also-corrupt-after-a-reboot-omv5-pi4-this-might-fix-it/