XDOCKER-101: Upgrade stable version to 11.2

* Added logic for configuring an external Solr instance using environment variables
* Updated README.md with instructions for using the XWiki image with an external Solr instance
* Added a script, contrib/solr/solr-init.sh, which initializes a Solr container to be usable by XWiki

* Improved docker-entrypoint.sh by adding compatibility for docker secrets
* Updated README.md with examples of using the XWiki image in Docker Swarm along with Docker secrets and Docker configs

http://www.xwiki.org/xwiki/bin/view/Documentation/AdminGuide/Installation/InstallationWAR/InstallationMySQL/

XDOCKER-40: Docker image use the wrong FS attachments setup

XDOCKER-40: Docker image use the wrong FS attachments setup

* Added explanations

* Implement a sync strategy while waiting for XWiki to support configurations files located in the permanent directory by default

* Use postgresql:9.5 since there's a problem with 9.6 (possibly an incompatibility with the driver version installed by libpostgresql-jdbc-java)

* Removed leftover

* More progress, still not working. I have an issue with "jdbc:postgresql://replacecontainer:5432/replacedatabase" not working and generating " Caused by: java.net.UnknownHostException: xwiki-postgres-db"
* Added ascii art in Dockerfile
* Renamed containers so that we can run several xwiki images on the same machine without conflict

* Use a better name for the property (thanks to Tianon)

* More progress, still not working

* Prepare the field by generifying the templates/build and docker files

 XDOCKER-10: Cannot change database name
* Add leftover that was forgotten

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