use a helper script as recommended on https://docs.docker.com/articles/dockerfile_best-practices/#entrypoint
Document way to create customized derived image with
additional plugins or hook scripts
user init.groovy.d directory so we can add more hook scripts
define JENKINS_OPTS so a derived image can customize jenkins launcher command line
defining some `ENV JENKINS_OPTS "..."`
Fix #21
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

default usage

	➜ docker run --rm -it jenkins 
	Running from: /usr/share/jenkins/jenkins.war
	webroot: EnvVars.masterEnvVars.get("JENKINS_HOME")
	Oct 07, 2014 12:06:48 PM winstone.Logger logInternal
	INFO: Beginning extraction from war file
    ...


detect jenkins launcher arguments

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

run alternate command

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