Add --no-pivot option for containers on ramdisk

This adds a `--no-pivot` cli flag to runc so that a container's rootfs
can be located ontop of ramdisk/tmpfs and not fail because you cannot
pivot root.

This should be a cli flag and not part of the spec because this is a
detail of the host/runtime environment and not an attribute of a
container.

Signed-off-by: Michael Crosby <crosbymichael@gmail.com>

utils.go

@@ -175,7 +175,12 @@ func createPidFile(path string, process *libcontainer.Process) error {
 }
 
 func createContainer(context *cli.Context, id string, spec *specs.Spec) (libcontainer.Container, error) {
-	config, err := specconv.CreateLibcontainerConfig(id, context.GlobalBool("systemd-cgroup"), spec)
+	config, err := specconv.CreateLibcontainerConfig(&specconv.CreateOpts{
+		CgroupName:       id,
+		UseSystemdCgroup: context.GlobalBool("systemd-cgroup"),
+		NoPivotRoot:      context.Bool("no-pivot"),
+		Spec:             spec,
+	})
 	if err != nil {
 		return nil, err
 	}