[abofh]

Yes, if you have constant inputs, it will produce mostly constant outputs (timestamps, etc).

But once you start getting time differential and caching, you run into... if not bugs, faults - your cached image came from a month ago, apt is now invalid. Sure, I should've used a private dpkg repository, but I want the side-effects from all of those.

A docker image I built today, is very likely to be ostensibly the same as the one I build in five minutes, but it doesn't guarantee that because its cache is at the image-level and the externalities (see: every possible step that isn't an ADD or COPY) is variadic.

It does a very valiant attempt at consistency to be clear, and I have my own gripes about it, but it's not like it creates a new planet from which technology can be built upon; It's more like creating a moon on which you can build a base, but you're still subject to the orbit of the earth.