Get rid of tomcat user for ease of use

I have tags `8-jre8` and `8-jre7`/`latest` building in an [automated build](https://registry.hub.docker.com/u/yosifkit/tomcat/) if someone can test them to ensure that their code works in it.

@bryantrobbins, or @jgangemi :pray:, think you could give it a quick test when you have a moment, especially in regards to our discussion about mounting into `$CATALINA_HOME/webapps` in #3.