Add xnat/docker-entrypoint.sh to handle tomcat container running as non-root

[westbywest]

As of 2d2a350, xnat/Dockerfile uses variables XNAT_UID / XNAT_GID to launch tomcat with arbitrary non-root user. If ./xnat-data/* folders don't already exist, docker-compose will create them with root ownership (designed docker behavior), making them inaccessible to xnat-web container.

A couple alternatives to simply requiring ./xnat-data/* folders exist ahead of time with appropriate ownership:

1. Always launch xnat-web container as root, but with xnat/docker-entrypoint.sh script to chown ./xnat-data/* folders and then launch tomcat as XNAT_UID.XNAT_GID. Postgres container does this: https://github.com/docker-library/postgres/blob/master/11/alpine/docker-entrypoint.sh
2. User alternate tomcat container like https://hub.docker.com/r/unidata/tomcat-docker