[Misc] Add missing license headers + apply docker best practice for consistency (see https://github.com/docker-library/official-images#consistency)

* 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] Spread ENV variables so that they are placed just before where they are used so that we can use Docker's build cache as aggressively as possible. Suggestion from Tianon Gravi.

Dockerfile optimisations (docker image size)

RUN chown -R tomcat8:tomcat8 /var/lib/tomcat8 directive is extremely costly, adding a layers "changing" only the owner 230MB of data, thus adding 230 MB needlesly

Removing a few layers to correct this

Gained

[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.

[Misc] Big overhaul to separate image-time setup from runtime configuration + added some license headers

[Misc] Improved README + give a name to the xwiki-data volume + add LICENSE + improved .gitignore file