Comment by kalmi10

I actually tried it on Docker 1.2.0 with the ubuntu:14.04 image.

/sys is already mounted and it is read-only, and it cannot be mounted manually:

  root@07ba8c752195:/# mkdir sys2
  root@07ba8c752195:/# mount -t sysfs sysfs /sys2
  mount: block device sysfs is write-protected, mounting read-only
  mount: cannot mount block device sysfs read-only