The multi-stage Dockerfile gotcha that bites people: the pg gem compiles against libpq-dev but needs libpq5 at runtime. Leave it out of the final stage and production crashes on boot. Nice side effect once the stages are split: Docker caches them independently, so an unchanged Gemfile skips the builder entirely. https://go.fastruby.io/zdn #Docker #RubyOnRails #DevOps