Improve docs: Add best practice for mount and unmount in user-callback

[aryoda]

I think the user-callback script in ~/.config/user-callback is called without root rights but me as user (if I have created a profile in the normal non-root Back in Time client).

It would be helpful for normal users to add an example into the docs and example script(s) which shows how to automatically mount a drive without root rights (or possible recommend "Back in Time (root)" for this use case)...

[aryoda]

More background info how the backintime scheduler works (for backintime started as normal user - NOT root!):

A user crontab entry is made so that the backintime scheduler is called by the current user (NOT root):

$ crontab -l
#Back In Time system entry, this will be edited by the gui:
0 11 * * * /usr/bin/nice -n19 /usr/bin/ionice -c2 -n7 /usr/bin/backintime backup-job >/dev/null

Since mount requires root rights I could add /usr/bin/mount to to the passwordless commands in the /etc/sudoers file
for me as user (see https://stackoverflow.com/a/25215640) but this would open a possible security whole by allowing an attacker that hijacks my login to mount other
drivers without the root password.

So what is the recommended best practice to mount a backup drive in the user-callback script?

[aryoda]

I successfully ended up creating and using backup profiles in Back In Time (root) since

• setting the SUID bit on a user-callback file for non-root BiT does not work (at least on Ubuntu which ignores the SUID bit on non-binary files like the user-callback bash script)
• enabling NOPASSWD in sudoers for the user-callback script is a security risk
• using a non-root mounting system for the userspace (like Gnome gio mount) may interfere with existing manual mounts (eg. different mounting points)

Any other ideas to be tested?

[aryoda]

Possibly related to bit-team/backintime#1229