Wondering why you think this is better. Not sure the trade off of a messy dockerfile and/or adding a bunch of layers (possibly bloating the image size) is worth the trade off if the concern is just about forgetting to update the dockerignore. The same could be said about gitignore.
Not the person you replied to, but personally, I like having control over what exactly gets into the final image, and (IME) have found that devs aren't great about remembering to update .dockerignore files. Re: extra layers, if you use multi-stage builds to separate the builder and final app images, you can avoid that.
