Bug 1088232 - mach ci-docker-run

I think this command came out of the necessity to have a local repo that could be cached between uses so that way when running the builder image or something we didn't have to clone each time.  

I don't think any cloning needs to be done within this command.  The user is responsible for having a valid hg clone somewhere and that is what will be mounted under /home/worker/x/source.  Now the one downside to this is that they will need to clone the repos themselves before doing anything.  If the source directory exists in the container (which it will when volume mounted even if the volume is empty) the builder script will not try to clone the repo.