cmd/ctr: create an image for checkpoint

This allows one to manage the checkpoints by using the `ctr image`
command.

The image is created with label "containerd.io/checkpoint". By
default, it is not included in the output of `ctr images ls`.
We can list the images by using the following command:
$ ctr images ls labels.containerd.\"io/checkpoint\"==true

Fixes #1026

Signed-off-by: Jacob Wen <jian.w.wen@oracle.com>

cmd/ctr/images.go

@@ -57,6 +57,16 @@ var imagesListCommand = cli.Command{
 		imageStore := client.ImageService()
 		cs := client.ContentStore()
 
+		if len(filters) == 0 {
+			filters = append(filters, `labels.containerd."io/checkpoint"!=true`)
+		} else {
+			for _, f := range filters {
+				if !strings.Contains(f, `labels.containerd."io/checkpoint"`) {
+					f += `,labels.containerd."io/checkpoint"!=true`
+					_ = f // ignore error: ineffectual assignment to f
+				}
+			}
+		}
 		imageList, err := imageStore.List(ctx, filters...)
 		if err != nil {
 			return errors.Wrap(err, "failed to list images")