[cpuguy83]

Right, they are integral but the user doesn't interact with them and they can be swapped out for a different implementation and the user would never know outside of noticing the impact that the change has (or trade-offs such a change would make).

Caching for `docker build` really doesn't have anything to do with layers except as a means of storage, it doesn't need to be layers at all.

To be honest, layers aren't even all that efficient (except in perfectly ideal scenarios), just better than no sharing at all and no one has taken the time to come up with something better yet...because frankly there are many other things that need improvements before bothering with it.

Again, just figured I'd mention again... since you are trying to explain Docker to me... I'm a Docker maintainer and have been for the last ~5 years. Happy to chat about this.