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/checkpoint.go

@@ -4,6 +4,7 @@ import (
 	"fmt"
 
 	"github.com/containerd/containerd"
+	"github.com/containerd/containerd/images"
 	"github.com/pkg/errors"
 	"github.com/urfave/cli"
 )
@@ -48,6 +49,19 @@ var taskCheckpointCommand = cli.Command{
 		if err != nil {
 			return err
 		}
+
+		labels := map[string]string{
+			"containerd.io/checkpoint": "true",
+		}
+		img := images.Image{
+			Name:   checkpoint.Digest.String(),
+			Target: checkpoint,
+			Labels: labels,
+		}
+		_, err = client.ImageService().Create(ctx, img)
+		if err != nil {
+			return err
+		}
 		fmt.Println(checkpoint.Digest.String())
 		return nil
 	},