Base Docker image on maven:3.8.2-jdk-11 and openjdk-11

Our Docker container builds on OpenJDK 8u232 released late in 2019, while the current LTS release of OpenJDK is 11.  Update our Dockerfile to OpenJDK 11 and address the following report from a user in a Github issue at the same time.

It would be great if the Docker image could be improved;

Drill documentation referrs to $DRILL_HOME environment variable which is not set in the Docker Image. Additionally in the doucmenation says it can be found in /etc/drill/conf while in the Docker image Drill has been installed into /opt/drill/conf

1. Add information how to add (mount) custom configuration files into the Docker image or even better, just make sure there is a Docker section in Drill's documentation whereever appropriate