shift bar buttons to left side in toolbar with swift

I have a toolbar that is placed at the bottom (correctly). The problem is the button (on a UIBarButtonItem) sits in the center of the entire toolbar.

How can I position this button to the side, but still (vertically) centered with the image? It'd be best with a margin.

So it'd look like

 --------------
|.    X
|.  label
 --------------

The code is:

    let customButton: UIButton = UIButton(type: .custom)
    customButton.setImage(UIImage(named: "start"), for: .normal)
    customButton.setTitle("Start", for: .normal)
    customButton.titleLabel?.font = UIFont.boldSystemFont(ofSize: 12)
    customButton.setTitleColor(UIColor.white, for: .normal)
    customButton.sizeToFit()
    customButton.centerLabelVerticallyWithPadding(spacing: 5)
    customButton.addGestureRecognizer(UITapGestureRecognizer(target: self, action: #selector(startButtonAction(_:))))
    iconBar.items = [UIBarButtonItem(customView: customButton)]

Where iconBar - the toolbar is defined as:

let iconBar: UIToolbar =
{
    let view = UIToolbar()
    view.translatesAutoresizingMaskIntoConstraints = false
    view.barTintColor = UIColor.black
    return view
}()

Also I'm using an extension to center the UIButton, then I can add an image (also centered) above it. Here's the extension:

extension UIButton
{
    func centerLabelVerticallyWithPadding(spacing:CGFloat)
    {
        // update positioning of image and title
        let imageSize = self.imageView!.frame.size
        self.titleEdgeInsets = UIEdgeInsets(top:0,
                                            left:-imageSize.width,
                                            bottom:-(imageSize.height + spacing),
                                            right:0)
        let titleSize = self.titleLabel!.frame.size
        self.imageEdgeInsets = UIEdgeInsets(top:-(titleSize.height + spacing),
                                            left:0,
                                            bottom: 0,
                                            right:-titleSize.width)

        // reset contentInset, so intrinsicContentSize() is still accurate
        let trueContentSize = self.titleLabel!.frame.union(self.imageView!.frame).size
        let oldContentSize = self.intrinsicContentSize
        let heightDelta = trueContentSize.height - oldContentSize.height
        let widthDelta = trueContentSize.width - oldContentSize.width
        self.contentEdgeInsets = UIEdgeInsets(top:heightDelta/2.0,
                                              left:widthDelta/2.0,
                                              bottom:heightDelta/2.0,
                                              right:widthDelta/2.0)
    }
}