I just started using the docker system.  What I noticed after installing my first docker container (App?) was that if I attempted to spin down my disks, one disk always came back up after about 2 seconds along with the parity drive. When I looked at that drive it had a share named "system" which I did not put there.  No other drive has that share.

 

The system share has a directory named "docker' and one named "libvirt".  The docker directory contains the file "docker.img" and the libvirt directory contains the file libvirt.img.  The docker.img file is getting updated constantly.

 

In my docker settings its states that the docker vdisk location is: /mnt/user/system/docker/docker.img

 

I am getting about 1000 writes per hour.  The only activity to my unraid disks are these write.  Here is a copy of my webgui main showing the high activity:

docker8.bmp

 

I have disabled the docker system in docker settings and stopped the docker container, but to no avail the activity (writes to the docker.img) keeps occurring.  

TLDR: Fixed in 6.9, but there's some tweaks you can do in 6.8 if you read through