I think you're not seeing the actual point. The review isn't solving the issue. If you have two branches with alternating environments and you switch back and forth between them, how do you suppose your Dockerfile would handle it? What if that change isn't as simple as switching the Ruby version, but "compile and install some software" or some obscure config changes? I mean, it would be breaking all of the time, you'd end up having two containers anyway and you'd be scratching your head as to how to switch between them easily.