Yeah, this is something I stressed at my place. Your Jeninksfile should set up environment variables, authentication related stuff, and call out to some build tool to build the project. The Jenkinsfile should also be configure to use a docker container to run the build within. In projects at my place that's a Docker file on the project that ultimately sets up and installs all the tools and dependencies required for a valid build environment that's just checked in along side the Jenkinsfile.