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Having /sys mounted inside the jail causes a lot of errors during jailkit cleanup:

https://www.howtoforge.com/community/threads/ispconfig3-2-1-var-log-ispconfig-cron-log-is-flooded-with-messages-14g.85824/

Also related, /proc is not included in jailkit directories. Possibly just remove /sys from those, or handle both /sys and /proc (and any others?) special (process for cleanup if regular directories, but ignore if they are mount points?).