How to apply a gradient to one edge of an image with ImageMagick?

Question

I'd like to add a white gradient to the bottom of an image with ImageMagick, going from the original image a few tens of pixels from the bottom to totally white at the bottom of the image; ideally, I'd like to be able to produce this given just the height of the gradient in pixels. I've looked at the ImageMagick documentation and tried to adapt answers to questions like this one, but I can't wrap my head around ImageMagick's syntax well enough to understand how to generate the gradient, let alone composite it on top of an existing image. How can I produce such an image?