Allow / as source of -v

We discussed this at the docker plumbers meetup and for tools and
working on the system for things like boot2docker and coreos this is
needed.  You can already bypass this check so we felt it is ok to start
allowing this feature.
Docker-DCO-1.1-Signed-off-by: Michael Crosby <michael@docker.com> (github: crosbymichael)

server/server.go

@@ -2055,19 +2055,11 @@ func (srv *Server) ContainerStart(job *engine.Job) engine.Status {
 	if len(job.Environ()) > 0 {
 		hostConfig := runconfig.ContainerHostConfigFromJob(job)
 		// Validate the HostConfig binds. Make sure that:
-		// 1) the source of a bind mount isn't /
-		//         The bind mount "/:/foo" isn't allowed.
-		// 2) Check that the source exists
-		//        The source to be bind mounted must exist.
+		// the source exists
 		for _, bind := range hostConfig.Binds {
 			splitBind := strings.Split(bind, ":")
 			source := splitBind[0]
 
-			// refuse to bind mount "/" to the container
-			if source == "/" {
-				return job.Errorf("Invalid bind mount '%s' : source can't be '/'", bind)
-			}
-
 			// ensure the source exists on the host
 			_, err := os.Stat(source)
 			if err != nil && os.IsNotExist(err) {