It is not easy to debug JFR when it's running in docker containers.

Similar to https://github.com/jenkinsci/docker/blob/master/jenkins.sh#L18-L23 , but for Jenkinsfile Runner

Acceptance criteria:

• There is an environment variable, which injects debug flags
• There is some documentation for it, e.g. https://github.com/oleg-nenashev/demo-jenkins-config-as-code#debugging-master