The way to do this with Capybara is documented on StackOverflow but, unfortunately, the answer there is buried in a little too much noise. I decided to create my own tiny noise-free blog post that contains the answer. Here it is:

the bloody mount points. I couldn't believe that when I realised what was going on. I got the wire brush and dettol out and scraped it off my drive. Never, ever again.

Plus, have you seen how many loopback mounting points it creates? "df" becomes very hard to use as it buries your actual drives with it's own. One for the daemon, one for GTK, one for Gnome, one for each of the snaps you have installed....