* Added comment

XDOCKER-131: docker-entrypoint doen't set database

* And switch to AdoptOpenJDK 8.x for XWiki 10.x (since XWiki 10.x doesn't support Java 11)

[Misc] Remove dead code since we don't release Docker images on XWiki 9.x anymore (LTS has moved to 10.x)

* 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

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

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

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

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

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

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

http://jira.xwiki.org/browse/XDOCKER-11

[Misc] Fix typo in docker-entrypoint.sh that prevented using a custom mysql user name + Removed unnecessary env variable at runtime

[Misc] Apply best practices as reported by yosifkit on https://github.com/docker-library/official-images/pull/2563#issuecomment-276541490
* Removed the not needed sh files
* Allow passing parameters to catalina.sh from the docker command line
* Removed unnecessary apt upgrade + unncessary --force-yes option
* Verify sha of xwiki war to make sure the right WAR is downloaded and avoid man in the middle attacks
* Simplify the sed expression and do it in one step
* Remove unnecessary EXPOSE (done by the tomcat image)
* Install the MySQL JDBC driver using apt-get (libmysql-java package) instead of getting it directly from upstream

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