Shortcut read-only bind-mounts in Windows, like non-Windows

Signed-off-by: Paul "TBBle" Hampson <Paul.Hampson@Pobox.com>

snapshot/localmounter_windows.go

@@ -25,6 +25,19 @@ func (lm *localMounter) Mount() (string, error) {
 		return "", errors.Wrapf(errdefs.ErrNotImplemented, "request to mount %d layers, only 1 is supported", len(lm.mounts))
 	}
 
+	if lm.mounts[0].Type == "bind" || lm.mounts[0].Type == "rbind" {
+		ro := false
+		for _, opt := range lm.mounts[0].Options {
+			if opt == "ro" {
+				ro = true
+				break
+			}
+		}
+		if !ro {
+			return lm.mounts[0].Source, nil
+		}
+	}
+
 	// Windows mounts always activate in-place, so the target of the mount must be the source directory.
 	// See https://github.com/containerd/containerd/pull/2366
 	dir := lm.mounts[0].Source