But those Docker images are not reproducible. They don't build everything from source from a trusted, well-known set of bootstrap binaries, nor will they produ

But those Docker images are not reproducible. They don't build everything from source from a trusted, well-known set of bootstrap binaries, nor will they produce bit-identical binaries. Docker does absolutely nothing to aid in the task of reproducible builds. See https://reproducible-builds.org https://reproducible-builds.org for more information about initiatives that are helping. Using something like GNU Guix instead of Docker, one could make good progress towards a reproducible Android tool chain that produces bit-identical APKs. Reproducibility is an ongoing problem, but Guix has been carefully designed to maximize reproducibility and to help identify what isn't reproducible.