docs: systemd unit example to mount root as a shared mount

[yehorb]

Hi.

Recently I decided to set up Distrobox on my (more-or-less) vanilla Ubuntu 24.04 host, and I ran into the "root filesystem is not mounted as a shared mount" problem.

The current compatibility page suggests to run mount --make-rshared / command and to add it to /etc/rc.local "to make it permanent". While mount --make-rshared / absolutely works, "add it to /etc/rc.local" does not. /etc/rc.local was not present on my system, and just creating it was not enough (it did nothing).

If I understand correctly, /etc/rc.local is considered deprecated (in Ubuntu, at least) in favor of systemd units, and to make it work you need to create some kind of rc-local.service - and there was no such service in my system.

If you need to interact with systemd anyway, you may as well create a proper service for the task. I believe this approach is more "in line" with what most distros consider "up to date".

Here I propose an addition to the compatibility page that explains how it can be done.

This solution is tested by me on my machine. But the service is quite simple, and I can't think of a reason why shouldn't it work on any other systemd-enabled system.

[dottorblaster]

Hi, thanks for this! The markdown linting check failed, could you check and update the guide accordingly?

[yehorb]

Looks like a simple formatting issue, should be good now. My local markdownlint is happy.

[dottorblaster]

Thank you!