docs: setcap (read-only `/`, root-like) setup

[jtagcat]

Continuation from https://github.com/restic/restic/issues/2408 and restic/docs/examples#backing_up_system_without_running_restic_as_root

It'd be nice if there was a similar example with resticprofile.

noping edit: I can /assign self, but due date I can promise would be 2022q2

[creativeprojects]

that's because I'm not using this setup. But I can try on one server and document it

[creativeprojects]

@jtagcat have you ever managed to get it working?

I was trying today without success. ~but then I realised that the code apparently has to elevate itself to the required capability: it's not transparent for the program~ I checked with a simple program and that's not the case, there's nothing to do in the code.

[jtagcat]

I have nothing to comment, I don't use resticprofile at the moment. Feel free to close.