XDOCKER-101: Upgrade stable version to 11.2

* Added explanations

* Implement a simple Gradle build to generate the various versions/variants.

* Fix typo from previous commit
* Remove the need for sudo and the need to execute as tomcat user
* Removed CATALINA_HOME and CATALINA_BASE from setenv.sh since they're not taken into account and copy setevn.sh to the right place
* Nano wasn't needed either (was a leftover)

[Misc] Go back to the previous setup which was better and I've found a way to make DockerHub happy with it.

[Misc] Since we may want to generate different images of xwiki in the future (with different dbs, different servlet containers, etc), move all current data into a xwiki-mysql-tomcat directory.