The assets pipeline compiles Javascript, CSS, images, fonts, and other static assets for use by the Rails application. In development, this compilation is allowed to happen at runtime; however, assets are required to be precompiled in deployed environments (RAILS_ENV=production) using the config.assets.compile = false option.
The assets pipeline mirrors the setup established by cssbundling-rails, which was adopted in Rails 7, and leverages sass compilation along with Sprockets.
All CSS must be imported by the application.sass.scss file. This is compiled by the build:css npm script into app/assets/builds/application.css.
The assets:precompile step in the Dockerfile takes care of everything else, compiling all linked assets in the Sprockets manifest.js file into the public/assets folder with digest strings embedded into each filename. This causes automatic browser cache-busting to ensure that users always receive the newest version of an asset after a deployment.
Read more about Sprockets here.