[kbolino]

That "entire point" is already accomplished by the built binary container image, which has a unique identifier in the form of its SHA-256 hash, and can be shared with others easily.

A reproducible build is grand, but somewhat tangential to that goal, and hard to obtain in practice. Besides the timestamp problem already mentioned, you can't always pin the versions of system libraries and other distribution-provided software. The large long-term cost of hosting and geographically distributing content leads to many distributions, and especially their externally provided package mirrors, discarding stale versions from repositories. Often, the only available versions are the one included in the release plus the latest N, with N sometimes as small as 1.

If you're building a no-frills image for production deployment of a single piece of software, this problem can be bypassed thanks to distroless and other stripped-down base images, but "batteries included" images can't go this route.