Practice of running `bundle exec rails build` inside a Docker image built from a clean Ruby base (plus cached heavy native deps like nokogiri) with explicit RAILS_ENV and NODE_ENV set to production. Avoids accidentally shipping dotfiles, development assets, or wrong-version bundles from the developer's laptop. A helper script extracts the resulting .gem file with `docker cp`, deleting any pre-existing local copy first so a failed build can't silently reuse an old artifact.