1.0-beta-16: Image size optimizations, new packaging flow

🚀 Image size improvements

• Exclude jenkins.war and executable-war.jar from Jenkinsfile Runner bundles, use JARs instead (#346) @oleg-nenashev
• Reduce Vanilla Docker image sizes by not bundling plugin HPI files (#349) @oleg-nenashev

Image optimization summary

👷 Changes for developers

• Add experimental Plugin Packaging POM for building custom Jenkinsfile Runner distributions (#352) @oleg-nenashev
  • This Maven-only flow is an addition to the existing Custom WAR Packager flow. In addition to Docker images, it allows packaging Jenkinsfile Runner executable JARs. The packaging takes less time, and the dependency conflicts resolution is much better.
  • Usage examples: Vanilla image, ci.jenkins.io-runner

📝 Documentation updates

• Add chat and mailing list links to README and Contributing guidelines (#354) @oleg-nenashev
• Update the Custom WAR Packager demos to use the new plugins and the Jenkins core (#348) @oleg-nenashev
• Switch back to GitHub Issues as a main bugtracker (#355) @oleg-nenashev