That makes more sense, but the deploy cycle is already really fast with Heroku. I'm not sure if there is very much value in trying to shave off a few more seconds.

Depending on your requirements build ties in Heroku can be quite long. One app i’ve recently observed take 7 minutes to build because it always had to build both JS and Django parts(buildpacks for both were executed every time). We made it way faster with custom docker file that used caches and often built under 40 seconds. i am not saying that is not doable in Heroku, but would take more effort.