iOS: How to set border for masked UIImageView

Question

I want to show image in message bubble. code:

class ViewController: UIViewController
{
    @IBOutlet weak var imageView: UIImageView!

    override func viewDidLoad()
    {
        super.viewDidLoad()
    }

    override func viewDidAppear(animated: Bool)
    {
        getMaskView()
    }

    func getMaskView()
    {
        let maskView = UIImageView(frame: CGRectMake(0, 0, 200, 200))
        maskView.image = UIImage(named: "BubbleGreen")?.stretchableImageWithLeftCapWidth(20, topCapHeight: 15)
        imageView.layer.mask = maskView.layer
    }
}

The result is:

Now, I want to set border for this bubble, for example, width = 2, color = red. I tried many methods, but failed anyway. Any solutions?