In the simple case, yes it is, but it scales poorly; consider ex. https://github.com/docker-library/postgres/blob/b9c080857b88... which is 112 lines run together like that to keep things in a single layer. This is pretty bad for readability. Also, as sibling comment notes, that only works for RUN commands, not other or mixed groups of commands.

It is, but the syntax is uglier. Also imagine that you might want to do more than just RUN in a single layer. Maybe COPY some file into the container and then RUN something that uses it.

Spend some time on docker hub and you'll find thousands of Dockerfiles with RUN <150 lines of && concatentated scripts>. It's dysfunctional.

Good to know, I often wondered why people produced these giant RUN statements instead of making multiple RUN.