Mention different "compose start" for package-based and image-based composes

[flozanorht]

The lecture tries to be generic of image builder, which can build edge and non-edge images, but there are commands which work only for edge images, while other commands only work for non-edge (package-based) images. This is an important concept and unintuitive, should be introduced by the lecture instead of only seen (and likely not noticed) during a lab.