That's absolutely no different than just installing and not re-installing. Docker adds nothing in this case.
Not re-building doesn't make your build reproducible. It just means that you're... not building. If I save the result of a single iteration of an RNG, I can't claim that the RNG always produces the same result because I saved the result somewhere...
Where did they say they’re not building? Building your app does not mean you install the modules every time. Some apps are so large they have to be split into chunks / layers anyway. In golang this used to be the way you’d add deps, check the entire source into your version control.
The code being the same != reproducible. Build tools can incorporate e.g. build timestamps into the built artifacts, or randomize the output for e.g. pattern scanning/patch deterrence.
The input is irrelevant. I think you should have a look at what reproducible builds really are before evangelizing them.
