Ensure JENKINS_HOME is owned by jenkins user

[carlossg]

If a volume is mounted, set the correct user permissions before launching Jenkins

As seen in Issue #11

[michaelneale]

@tianon any feedback on validity of doing a CHOWN to bind mounted volumes like this in an official image?

[tianon]

There's definitely already precedent for it in some of the database images; however, su has very nasty behavior when it comes to proper signal handling and even some odd TTY-related issues, so I would really highly discourage its use for this purpose.

In the other images, we've used a tiny utility I developed specificaly for this called gosu (https://github.com/tianon/gosu). See https://github.com/docker-library/postgres/blob/master/Dockerfile.template#L9-L10 and https://github.com/docker-library/postgres/blob/master/docker-entrypoint.sh#L15 for examples from the Postgres image.

[carlossg]

You are right, better be conservative in the base image. Maybe just add a check that the dir can be written instead of the stack trace that jenkins throws would be helpful