monit check disk size is not thrustable with docker layers

[JeGoi]

Describe the bug
Monit is managing disk space issues but the script is not able to handle docker overlayers.
Docker root folder may contain files from other folders, such as mounted to merged, if you test the disc usage with or without "-x" of du (or ncdu), it will exclude those files out and the reduction is drastic.
So, monit is lying with the current disk usage. du -x is better and df is giving something not that bad.

To Reproduce
Steps to reproduce the behavior:

1. rebuild several images at several point of time (like every 3 month
2. do: du -h -d1 -x / , du -h -d1 / and df -h you will see difference in disk size available.

Screenshots
ncdu without -x

ncdu with -x

and df

Expected behavior
Having a clear (better view) of disk space available and send email only when really needed.