Centralize the build flow in this repo

[oleg-nenashev]

Currently the repo uses a custom Docker image, which is produced in another repository (and which is quite outdated). Once https://issues.jenkins-ci.org/browse/JENKINS-56233 is implemented, we can automate the build of based Docker images in this repo.

• Move Custom WAR Packager definitions to the repo so that it can produce the base Jenkinsfile Runner image for GitHub actions
• Consider setting up a Dependabot-powered update flow like in https://github.com/jenkinsci/ci.jenkins.io-runner