VyOS makes use of Jenkins as our Continuous Integration (CI) service. Our CI server is publicly accessible here: https://ci.vyos.net. You can get a brief overview of all required components shipped in a VyOS ISO.
To build our modules we utilize a CI/CD Pipeline script. Each and every VyOS component comes with it's own Jenkinsfile which is (more or less) a copy. The Pipeline utilizes the Docker container from the Build ISO section - but instead of building it from source on every run, we rather always fetch a fresh copy (if needed) from Dockerhub.
Each module is build on demand if a new commit on the branch in question is found. After a successful run the resulting Debian Package(s) will be deployed to our Debian repository which is used during build time. It is located here: http://dev.packages.vyos.net/repositories/.