Set up reproducible build infrastructure

[aakselrod]

Now that we're building packages, we should ensure that we can build cross-platform binaries reproducibly on multiple platforms. It would be good to support building on at least amd64 (PC) and arm64/v8 (RPi 4, Apple m1), and also build for at least riscv/riscv64 and arm/v6 (older RPi versions). In addition, it would be good to add Woodpecker-CI support for replicated builds on local instances of Forgejo/Gitea and/or Codeberg at the same time.