Allow running on the Openshift platform

[woopla]

A few changes are necessary for the best out-of-the-box experience on Docker-based application
platforms such as OpenShift:

1. For security purposes, they run the containers as non-root. Before this change, the user was
   trying to write to the home dir, which is set to /root by default.
2. Deploy time is limited, and the download of dependencies by Jetty at start-up time, while
   convenient for testing, causes timeouts when deploying. a WAR is now created during
   Docker image creation, and as a result starting up the container is really fast now.

In addition, it is now possible to enable debug logging for Jetty by uncommenting the
relevant line in the Dockerfile.

[arnaudroques]

I had to (temporary?) rename this Dockerfile to Dockerfile.openshift because we are currently trying to create an official docker image (see #40 )
When the official docker will be working, we'll see how to merge Dockerfiles.
Sorry about that!

[woopla]

Hmmm actually I'm going to try the modified Dockerfile from #40, I think it should be possible to minimally modify it to get it running on Openshift. Openshift can also pull images from Docker registries directly, so this would be helpful.

My goal is to make it Openshift-compatible, not Openshift-exclusive: the changes I'll make should not change anything for regular Docker users.

[woopla]

So I looked at the new Dockerfile, and the problem I have is that it expects a WAR file to be there. This won't work for regular users who want to use the Dockerfile. I would actually do the following:

1. Rename Dockerfile to Dockerfile.travis
2. Modify the docker build line in .travis.yml to docker build -t plantuml/plantuml-server:latest -f Dockerfile.travis . ;
3. Rename Dockerfile.openshift to Dockerfile

This way everything should work out of the box for everybody.

My .02,

Clément