This is a simple example of extending the docker-lbrary/tomcat image with your own files and configurations.
For more details on the tomcat image, you can also view its page on Docker Hub.
The Dockerfile here defines a new image, "bryantrobbins/docker-tomcat-example", which starts from a base of the tomcat:8.0 image. It then adds a tomcat-users.xml file to the Tomcat install location. You can do things similar to this to stage any such static files that would be constant across your running Tomcat containers.
As a tradeoff, we must build this image first, and use this image's details when starting our Tomcat containers.
I've also included "build.sh" and "run.sh" scripts as examples of how to build and run this image. The first time that you build the image, a number of dependencies will be fetched and cached locally for quicker builds next time.
You can take a look at these bash scripts to see the full docker commands I've used.