Is diffing images really necessary? I've done multi step builds in the past for a Java app and I didn't find it especially hard to only include what was required. E.g. the final resultant image would exclude maven, which would be used in the first stage of the build.
If you copy over files directly built by your build steps, sure, you don't really need diffing images, you know where things are. If you're trying to build a minimal image with some specific dependencies copied over that are the result of things you have less control over diffing is pretty much the only way to figure out what you need IME. E.g. you want to copy only the parts of dependencies you actually need.
Multi Step Images will not help if you want to build a python container and the package manager will pull in e.b. Bash for the python runtime snd many more depencies. It only saves you from adding build tools to the image.
