Swift - Cut hole in shadow layer

I want to "cut a hole" in the shadow layer of a UIView an Swift3, iOS

I have a container (UIView), that has 2 children:

• one UIImageView
• one UIView on top of that image ("overlay")

I want to give the overlay a shadow and cut out an inner rect of that shadow, to create a glow-like effect at the edges of the ImageView
It is crucial that the glow is inset, since the image is taking the screen width
My code so far:

let glowView = UIView(frame: CGRect(x: 0, y: 0, width: imageWidth, height: imageHeight))
glowView.layer.shadowPath = UIBezierPath(roundedRect: container.bounds, cornerRadius: 4.0).cgPath
glowView.layer.shouldRasterize = true
glowView.layer.rasterizationScale = UIScreen.main.scale
glowView.layer.shadowOffset = CGSize(width: 1.0, height: 1.0)
glowView.layer.shadowOpacity = 0.4

container.addSubview(imageView)
container.addSubview(glowView)

The result looks like the following right now:

Now I would like to cut out the darker inner part, so that just the shadow at the edges remains
Any idea how to achieve this?

from Swift - Cut hole in shadow layer