Add commands to mount/unmount image from ref

Example:

```terminal
$ mkdir /opt/busybox
$ ctr image mount docker.io/library/busybox:latest /opt/busybox
/opt/busybox
$ ls -lh /opt/busybox
total 40K
drwxr-xr-x 2 root   root     12K Apr 14 01:10 bin
drwxr-xr-x 2 root   root    4.0K Apr 14 01:10 dev
drwxr-xr-x 3 root   root    4.0K Apr 14 01:10 etc
drwxr-xr-x 2 nobody nogroup 4.0K Apr 14 01:10 home
drwx------ 2 root   root    4.0K Apr 14 01:10 root
drwxrwxrwt 2 root   root    4.0K Apr 14 01:10 tmp
drwxr-xr-x 3 root   root    4.0K Apr 14 01:10 usr
drwxr-xr-x 4 root   root    4.0K Apr 14 01:10 var
$ ctr image unmount /opt/busybox
$ ls -lh /opt/busybox
total 0
```

Signed-off-by: Brian Goff <cpuguy83@gmail.com>

cmd/ctr/commands/images/images.go

@@ -43,6 +43,8 @@ var Command = cli.Command{
 		exportCommand,
 		importCommand,
 		listCommand,
+		mountCommand,
+		unmountCommand,
 		pullCommand,
 		pushCommand,
 		removeCommand,

cmd/ctr/commands/images/mount.go

@@ -0,0 +1,143 @@
+package images
+
+import (
+	"fmt"
+	"time"
+
+	"github.com/containerd/containerd"
+	"github.com/containerd/containerd/cmd/ctr/commands"
+	"github.com/containerd/containerd/errdefs"
+	"github.com/containerd/containerd/leases"
+	"github.com/containerd/containerd/mount"
+	"github.com/containerd/containerd/platforms"
+	"github.com/opencontainers/image-spec/identity"
+	"github.com/pkg/errors"
+	"github.com/urfave/cli"
+)
+
+/*
+   Copyright The containerd Authors.
+
+   Licensed under the Apache License, Version 2.0 (the "License");
+   you may not use this file except in compliance with the License.
+   You may obtain a copy of the License at
+
+       http://www.apache.org/licenses/LICENSE-2.0
+
+   Unless required by applicable law or agreed to in writing, software
+   distributed under the License is distributed on an "AS IS" BASIS,
+   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+   See the License for the specific language governing permissions and
+   limitations under the License.
+*/
+
+var mountCommand = cli.Command{
+	Name:      "mount",
+	Usage:     "mount an image to a target path",
+	ArgsUsage: "[flags] <ref> <target>",
+	Description: `Mount an image rootfs to a specified path.
+
+When you are done, use the unmount command.
+`,
+	Flags: append(append(commands.RegistryFlags, append(commands.SnapshotterFlags, commands.LabelFlag)...),
+		cli.BoolFlag{
+			Name:  "rw",
+			Usage: "Enable write support on the mount",
+		},
+		cli.StringFlag{
+			Name:  "platform",
+			Usage: "Mount the image for the specified platform",
+			Value: platforms.DefaultString(),
+		},
+	),
+	Action: func(context *cli.Context) (retErr error) {
+		var (
+			ref    = context.Args().First()
+			target = context.Args().Get(1)
+		)
+		if ref == "" {
+			return fmt.Errorf("please provide an image reference to mount")
+		}
+		if target == "" {
+			return fmt.Errorf("please provide a target path to mount to")
+		}
+
+		client, ctx, cancel, err := commands.NewClient(context)
+		if err != nil {
+			return err
+		}
+		defer cancel()
+
+		snapshotter := context.GlobalString("snapshotter")
+		if snapshotter == "" {
+			snapshotter = containerd.DefaultSnapshotter
+		}
+
+		ctx, done, err := client.WithLease(ctx,
+			leases.WithID(target),
+			leases.WithExpiration(24*time.Hour),
+			leases.WithLabels(map[string]string{
+				"containerd.io/gc.ref.snapshot." + snapshotter: target,
+			}),
+		)
+		if err != nil && !errdefs.IsAlreadyExists(err) {
+			return err
+		}
+
+		defer func() {
+			if retErr != nil && done != nil {
+				done(ctx)
+			}
+		}()
+
+		ps := context.String("platform")
+		p, err := platforms.Parse(ps)
+		if err != nil {
+			return errors.Wrapf(err, "unable to parse platform %s", ps)
+		}
+
+		img, err := client.ImageService().Get(ctx, ref)
+		if err != nil {
+			return err
+		}
+
+		i := containerd.NewImageWithPlatform(client, img, platforms.Only(p))
+		if err := i.Unpack(ctx, snapshotter); err != nil {
+			return errors.Wrap(err, "error unpacking image")
+		}
+
+		diffIDs, err := i.RootFS(ctx)
+		if err != nil {
+			return err
+		}
+		chainID := identity.ChainID(diffIDs).String()
+		fmt.Println(chainID)
+
+		s := client.SnapshotService(snapshotter)
+
+		var mounts []mount.Mount
+		if context.Bool("rw") {
+			mounts, err = s.Prepare(ctx, target, chainID)
+		} else {
+			mounts, err = s.View(ctx, target, chainID)
+		}
+		if err != nil {
+			if errdefs.IsAlreadyExists(err) {
+				mounts, err = s.Mounts(ctx, target)
+			}
+			if err != nil {
+				return err
+			}
+		}
+
+		if err := mount.All(mounts, target); err != nil {
+			if err := s.Remove(ctx, target); err != nil && !errdefs.IsNotFound(err) {
+				fmt.Fprintln(context.App.ErrWriter, "Error cleaning up snapshot after mount error:", err)
+			}
+			return err
+		}
+
+		fmt.Fprintln(context.App.Writer, target)
+		return nil
+	},
+}

cmd/ctr/commands/images/unmount.go

@@ -0,0 +1,73 @@
+package images
+
+import (
+	"fmt"
+
+	"github.com/containerd/containerd/cmd/ctr/commands"
+	"github.com/containerd/containerd/errdefs"
+	"github.com/containerd/containerd/leases"
+	"github.com/containerd/containerd/mount"
+	"github.com/pkg/errors"
+	"github.com/urfave/cli"
+)
+
+/*
+   Copyright The containerd Authors.
+
+   Licensed under the Apache License, Version 2.0 (the "License");
+   you may not use this file except in compliance with the License.
+   You may obtain a copy of the License at
+
+       http://www.apache.org/licenses/LICENSE-2.0
+
+   Unless required by applicable law or agreed to in writing, software
+   distributed under the License is distributed on an "AS IS" BASIS,
+   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+   See the License for the specific language governing permissions and
+   limitations under the License.
+*/
+
+var unmountCommand = cli.Command{
+	Name:        "unmount",
+	Usage:       "unmount the image from the target",
+	ArgsUsage:   "[flags] <target>",
+	Description: "Unmount the image rootfs from the specified target.",
+	Flags: append(append(commands.RegistryFlags, append(commands.SnapshotterFlags, commands.LabelFlag)...),
+		cli.BoolFlag{
+			Name:  "rm",
+			Usage: "remove the snapshot after a successful unmount",
+		},
+	),
+	Action: func(context *cli.Context) error {
+		var (
+			target = context.Args().First()
+		)
+		if target == "" {
+			return fmt.Errorf("please provide a target path to mount to")
+		}
+
+		client, ctx, cancel, err := commands.NewClient(context)
+		if err != nil {
+			return err
+		}
+		defer cancel()
+
+		if err := mount.UnmountAll(target, 0); err != nil {
+			return err
+		}
+
+		if context.Bool("rm") {
+			snapshotter := context.String("snapshotter")
+			s := client.SnapshotService(snapshotter)
+			if err := client.LeasesService().Delete(ctx, leases.Lease{ID: target}); err != nil && !errdefs.IsNotFound(err) {
+				return errors.Wrap(err, "error deleting lease")
+			}
+			if err := s.Remove(ctx, target); err != nil && !errdefs.IsNotFound(err) {
+				return errors.Wrap(err, "error removing snapshot")
+			}
+		}
+
+		fmt.Fprintln(context.App.Writer, target)
+		return nil
+	},
+}