Not sure why you are coming here to rant about "a lot of people complaining about Dockerfile being too complicated" when the topic here is a gotcha that's not a

Not sure why you are coming here to rant about "a lot of people complaining about Dockerfile being too complicated" when the topic here is a gotcha that's not at all obvious and Docker working in a way that is not intuitive. If I specify ARG I obviously expect it to be set everywhere in the Dockerfile. Your rant is out of place because it generalizes and attempts to shift topics.