At day 2 of postmarketOS I've played with the thought of implementing a reproducible binary repository. I have put a lot of effort into it, and it partially worked. But overall this slows down postmarketOS development and doesn't have a real benefit right now (mostly because Alpine isn't reproducible itself). But I still think that this is cool technology and that we should get there at some point.
After that came the second approach for a (non-reproducible) binary repository, that has actually been implemented and is what we have right now.
I've archived this git repository, because the issues document a few reproducibility issues and the wiki outlines the plan in general. That might come in handy at some later point.