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

  * Missing update of docker-compose so that the build don't put back
    12.1-alpine

* Also update the compose files

* 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)

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)