Yours isn't overly complex, it's one way to trim down an image. However, it is a lot more complicated than just defining 1 Dockerfile that at least copies in your package.json file separately to speed up future deploys that don't touch your packages.

I guess I just don't see the time vs. effort value in optimizing most smaller projects.

For example, that 75 gem project may take 5 minutes to build once but after that it takes 10 seconds to build and push a version that updates the app code.

I'm ok with this pattern for most of my projects because you can easily get by with 1 host to serve tens of thousands of requests a month on most typical projects. It's not like I'm spinning up and destroying dozens of instances a day where the network overhead is a legit concern (if I were, then I would optimize).