Update user guide with interesting docker use case

[MichaelWedel]

This fixes #258.

I've tested this locally by copying one of the example devices to another directory on the host and then following the newly added instructions.

As written in the ticket, credit for the idea goes to John Sparger.

[MikeHart85]

Looks good, ready to merge?

As a sidenote, this reminds me how much I don't like the way -a/-k works. I've made a separate issue for that: #260.