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