Use image constructor in client

Replace manual image struct creation with the image
constructor which is there to do just that.

Signed-off-by: Derek McGowan <derek@mcgstyle.net>

container.go

@@ -173,10 +173,7 @@ func (c *container) Image(ctx context.Context) (Image, error) {
 	if err != nil {
 		return nil, errors.Wrapf(err, "failed to get image %s for container", r.Image)
 	}
-	return &image{
-		client: c.client,
-		i:      i,
-	}, nil
+	return NewImage(c.client, i), nil
 }
 
 func (c *container) NewTask(ctx context.Context, ioCreate cio.Creator, opts ...NewTaskOpts) (_ Task, err error) {