client: add Import() and Export() for importing/exporting image in OCI format

Export as a tar (Note: "-" can be used for stdout):

    $ ctr images export /tmp/oci-busybox.tar docker.io/library/busybox:latest

Import a tar (Note: "-" can be used for stdin):

    $ ctr images import foo/new:latest /tmp/oci-busybox.tar

Note: media types are not converted at the moment: e.g.
  application/vnd.docker.image.rootfs.diff.tar.gzip
  -> application/vnd.oci.image.layer.v1.tar+gzip

Signed-off-by: Akihiro Suda <suda.akihiro@lab.ntt.co.jp>

cmd/ctr/import.go

@@ -0,0 +1,70 @@
+package main
+
+import (
+	"fmt"
+	"io"
+	"os"
+
+	"github.com/containerd/containerd"
+	"github.com/containerd/containerd/log"
+	"github.com/urfave/cli"
+)
+
+var imagesImportCommand = cli.Command{
+	Name:      "import",
+	Usage:     "import an image",
+	ArgsUsage: "[flags] <ref> <in>",
+	Description: `Import an image from a tar stream
+`,
+	Flags: []cli.Flag{
+		cli.StringFlag{
+			Name:  "ref-object",
+			Value: "",
+			Usage: "reference object e.g. tag@digest (default: use the object specified in ref)",
+		},
+	},
+	Action: func(clicontext *cli.Context) error {
+		var (
+			ref       = clicontext.Args().First()
+			in        = clicontext.Args().Get(1)
+			refObject = clicontext.String("ref-object")
+		)
+
+		ctx, cancel := appContext(clicontext)
+		defer cancel()
+
+		client, err := newClient(clicontext)
+		if err != nil {
+			return err
+		}
+
+		var r io.ReadCloser
+		if in == "-" {
+			r = os.Stdin
+		} else {
+			r, err = os.Open(in)
+			if err != nil {
+				return err
+			}
+		}
+		img, err := client.Import(ctx,
+			ref,
+			r,
+			containerd.WithRefObject(refObject),
+		)
+		if err != nil {
+			return err
+		}
+		if err = r.Close(); err != nil {
+			return err
+		}
+
+		log.G(ctx).WithField("image", ref).Debug("unpacking")
+
+		// TODO: Show unpack status
+		fmt.Printf("unpacking %s...", img.Target().Digest)
+		err = img.Unpack(ctx, clicontext.String("snapshotter"))
+		fmt.Println("done")
+		return err
+	},
+}