fix --read-only containers under --userns-remap

The documentation here:
https://docs.docker.com/engine/security/userns-remap/#user-namespace-known-limitations

says that readonly containers can't be used with user namespaces do to some
kernel restriction. In fact, there is a special case in the kernel to be
able to do stuff like this, so let's use it.

This takes us from:

ubuntu@docker:~$ docker run -it --read-only ubuntu
docker: Error response from daemon: oci runtime error: container_linux.go:262: starting container process caused "process_linux.go:339: container init caused \"rootfs_linux.go:125: remounting \\\"/dev\\\" as readonly caused \\\"operation not permitted\\\"\"".

to:

ubuntu@docker:~$ docker-runc --version
runc version 1.0.0-rc4+dev
commit: ae29480-dirty
spec: 1.0.0
ubuntu@docker:~$ docker run -it --read-only ubuntu
root@181e2acb909a:/# touch foo
touch: cannot touch 'foo': Read-only file system

Signed-off-by: Tycho Andersen <tycho@docker.com>

libcontainer/rootfs_linux.go

@@ -733,7 +733,14 @@ func remountReadonly(m *configs.Mount) error {
 		flags = m.Flags
 	)
 	for i := 0; i < 5; i++ {
-		if err := unix.Mount("", dest, "", uintptr(flags|unix.MS_REMOUNT|unix.MS_RDONLY), ""); err != nil {
+		// There is a special case in the kernel for
+		// MS_REMOUNT | MS_BIND, which allows us to change only the
+		// flags even as an unprivileged user (i.e. user namespace)
+		// assuming we don't drop any security related flags (nodev,
+		// nosuid, etc.). So, let's use that case so that we can do
+		// this re-mount without failing in a userns.
+		flags |= unix.MS_REMOUNT | unix.MS_BIND | unix.MS_RDONLY
+		if err := unix.Mount("", dest, "", uintptr(flags), ""); err != nil {
 			switch err {
 			case unix.EBUSY:
 				time.Sleep(100 * time.Millisecond)