Stop using Docker link feature (legacy/deprecated)

Xwiki official docker image relies on Docker "link" feature which is deprecated.
https://docs.docker.com/engine/userguide/networking/default_network/dockerlinks/

docker run --name xwiki -p 8080:8080 -v /my/own/xwiki:/usr/local/xwiki -e MYSQL_USER=xwiki -e MYSQL_PASSWORD=xwiki -e MYSQL_DATABASE=xwiki --link mysql-xwiki:db xwiki:mysql-tomcat

The workaround would be to just use the Docker networks instead of "link" but his cannot work as it is because docker image relies on the fact that the database container will always be called "db" in hibernates file :
jdbc:mysql://db/${MYSQL_DATABASE:-xwiki}?useSSL=false

To make the switch, you should:

• change this line to "jdbc:mysql://replacehostname/replacedbname?useSSL=false"
• add sed replacements in "function configure()" of the entrypoint
• adapt Docker-compose files
• change the README of course

If you are interested I'll propose a PR