Run as non-root by default

[md5]

Like most of the Docker official images, this image is currently configured to run as root. I'm opening this issue as a place to discuss what change should be made, if any. See this discussion in the context of the tomcat image for the sort of issues that can be anticipated for jetty.

I can see a few changes/enhancements being made to this image:

1. Add code to the Dockerfile to create a consistent jetty user to allow users to easily do USER jetty in a derived Dockerfile or -u jetty in their docker run command.
2. Document the use of JETTY_BASE for the jetty:9 image and give an example that uses --module setuid to drop privileges to the jetty user after starting as root.