clean up and add Volume
Show how to customise
tidy up by moving clean
Add a few minor tweaks and reorganizations to improve caching and overall build
Merge pull request #12 from tianon/minor-tweaks

Add a few minor tweaks and reorganizations to improve caching and overall build
Use Java official image as base
option to set JVM parameters, especially system properties
dowload jenkins.war, no need for debian package
the later include daemon setup that doesn’t make sense here
« ADD » doesn’t use Last-Modified header so download on every build
Merge branch 'war'
LTS is 1.565.3
Prevent copying the jenkins war in two different layers
Merge pull request #15 from carlossg/size

Reducing size
Can’t declare an empty env variable
just don’t declare it
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:/$
Set default Jenkins max open files

Signed-off-by: Sam Xiao <sam.xs.xiao@gmail.com>