Bug 1088232 - mach ci-docker-run

So I think we have a choice...  Either the user needs a valid hg repo already that will be mounted under /home/worker/x/source with this command (which I think how it's written now is this option) -or-
they just provide a higherlevel m-c or gaia directory that will be used for cloning that's empty and since the 'source' directory doesn't exist there, the builder image will clone to it the first time it's run. 

I personally prefer the first option since it's very straight forward to the user as to what's happening, but I'm up for options that will make this less painful for everyone.