Conform to [dockerfile best practices](https://docs.docker.com/articles/dockerfile_best-practices/#entrypoint) to let user run alternate command
but still detect user passing jenkins launcher custom arguments

	➜ docker run --rm -it jenkins --version
	Running from: /usr/share/jenkins/jenkins.war
	webroot: EnvVars.masterEnvVars.get("JENKINS_HOME")
	1.565.3

    ➜ docker run --rm -it jenkins bash
	jenkins@24c6215c8492:/$