Allow not running Tomcat as root

Putting this as an idea since I don't know if this is wanted or not.

By default the official tomcat docker image at https://hub.docker.com/_/tomcat (which is the image we depend on for our xwiki docker image) installs Tomcat as root.

If you wish to not run it as root, you need to change the permissions in the xwiki docker image for the tomcat files at /usr/local/tomcat, as mentioned on https://gist.github.com/desaster/c0af97481b5dbeecd57286ba030715f7 , and save the modified container as a new image.

If we think it’s a problem, we could also fix the permissions and add a tomcat user in our xwiki dockerfile.

See also https://github.com/docker-library/tomcat/issues/147 & https://github.com/docker-library/tomcat/issues/68#issuecomment-311745802