In general, I think of multi-stage builds as if multiple docker files were concatenated; if ARG or SHELL doesn't exist in the theoretical split Dockerfile, then it wouldn't exist in the multistage build.

Docker(file) is a thing which brought in some quite neat new ideas, and a lot of these "footguns". I did read the docs, and I am comfortable now building even complicated "Dockerfile-architectures" with proper linting, watching out for good caching, etc. I wish there were better alternatives; I saw images being built with Bazel, and thanks, but that is even more convoluted, with presumably a different set of footguns.

I am not saying that Dockerfiles are good. But I also think that knowing the tool you use differentiates you from being a rookie to an experienced professional.

I agree in general, though I'd also add that a tool requiring somebody to know and avoid a large number of footguns is what differentiates a tool from being a prototype and being production-ready. While Docker has a large number of features, the number of gotchas makes it feel like something that should only be used in hobby projects, never in production.

In the end, I ended up making several scripts to automate the generation and execution of dockerfiles for my common use cases. I trust myself to proceed cautiously around the beartraps once. I don't trust myself to sprint through a field of beartraps if there's a short deadline.