Describe the PR

• This enhances the current release workflow to also trigger the workflow in https://github.com/pmd/docker to create a new image
• This is a workflow in a different repository, so GitHub Actions permissions are bit different. In order to make this (hopefully) work, I created a new custom GitHub App in our organization (https://github.com/pmd) and installed it. This GitHub App is named "PMD Actions Helper" and for now has the only permission to trigger workflows in any of our repositories (permission: actions=write).
• This app installation is used to request a new installation access token when the release workflow is run and this token is used to execute workflow dispatch API call (https://docs.github.com/en/rest/actions/workflows?apiVersion=2022-11-28#create-a-workflow-dispatch-event )
• The app id and private key are configured as organization wide secrets

Related issues

• Fix Maintain a public PMD docker image #5448

Ready?

• Added unit tests for fixed bug/feature
• Passing all unit tests
• Complete build ./mvnw clean verify passes (checked automatically by github actions)
• Added (in-code) documentation (if needed)