There are few examples when a Custom Docker Image is required to bundle tools into Jenkinsfile Runner and classic Docker images produced by Custom WAR Packager:

• https://github.com/jenkinsci/ci.jenkins.io-runner/blob/master/Dockerfile (adding Maven)
• https://github.com/jenkinsci/jenkinsfile-runner-github-actions/blob/master/Dockerfile#L1 - Custom image, with Maven again. CC Johannes Nicolai
• https://github.com/jenkins-x/jenkins-x-serverless - creating multiple custom images on the top of the JFR-based image

We could simplify all the use-cases by allowing to inject some some custom Docker steps from packager-config.yml