Give context for the error that occures when you try to bind mount a vol...

...ume with a relative path

I ran into this error here https://github.com/timthelion/qubes-tools-debian/issues/1 and it took me a rather long time to figure out what was going on.

Docker-DCO-1.1-Signed-off-by: Timothy <timothyhobbs@seznam.cz> (github: timthelion)