A 12-factor app is a methodology or set of principles for building scalable and performant, independent, and most resilient enterprise applications.
12-factor app principles got very popular as it aligns with Microservice principles.
Release stage: get the build package from the build stage and combines with the configurations of the deployment environment and make your application ready to run.
The twelve-factor app is completely self-contained and doesn’t rely on runtime injection of a webserver into the execution environment to create a web-facing service.