code: func WithUpgrade(i containerd.Image) containerd.UpdateContainerOpts {
  	return func(ctx context.Context, client *containerd.Client, c *containers.Container) error {
  		revision, err := save(ctx, client, i, c)
  		if err != nil {
  			return err
  		}
  		c.Image = i.Name()
  		c.SnapshotKey = revision.Key
  		return nil
  	}
  }
docs: // WithUpgrade upgrades an existing container's image to a new one