(right now, specifically only for Docker, but could potentially be used
for any crazy container impl where the local filesystem doesn't appear
local to df -l.  From the commit:)

  Some environments do not give us a valid / mount (like Docker); thus,
  df -l does not work.  Thus we must rely on /proc/mounts to tell us if
  a dir is local or not.  Well, ok, /proc/mounts is hard (and potential
  bind mount chains would make it harder).  So instead, we assume that /
  is local, (well, we check to ensure it is not NFS in /proc/mounts),
  and use os_samefs above to ensure that $dir is on the same device as
  /.  If that is true, that is good enough to call it a local dir.