Photoshop masking: Drawing a shape on top of an image, and masking what is outside the shape

Photoshop masking: Drawing a shape on top of an image, and masking what is outside the shape

I have an image that I want to turn into a button. I want to draw a rounded rectangle on top of the image. I want to then be able to see the image inside the button, and outside of the button, I want transparent pixels.