I mean, you can do both. Or, technically with that link you mentioned, all three.
You can use HEREDOCS to combo together commands that make sense in a layer, ensure your layers are ordered such that the more-frequently changing ones are further on in your Dockerfile when possible (this will also speed up your builds, ensuring as many caches as possible are more likely to be valid), and use mutli-stage builds on top of that to really pare it down to the bare necessities.
You can use HEREDOCS to combo together commands that make sense in a layer, ensure your layers are ordered such that the more-frequently changing ones are further on in your Dockerfile when possible (this will also speed up your builds, ensuring as many caches as possible are more likely to be valid), and use mutli-stage builds on top of that to really pare it down to the bare necessities.