It's not necessary about the size, and nor is it always a premature/useless optimization. It's about knowing how layers interact which is important. And not hard to teach either.
And yes, if you're adding 5+ GB to each image build through something silly like a 'RUN chown' it's worth fixing. Maybe not complex refactoring, sure, but we are talking about the basics here.
And yes, if you're adding 5+ GB to each image build through something silly like a 'RUN chown' it's worth fixing. Maybe not complex refactoring, sure, but we are talking about the basics here.