The documentation refered to the '11' subdirectory which no longer
exists. Fix this by using the current '12' instead.

* Fix syntax

The documentation was not entirely clear about the need to create directories for bind mounts for the database and application containers, so I've added some context hear for the avoidance of doubt.

I've also added a warning in each case to ensure that fully-qualified directory names are passed.  If you pass a multi-level relative path, you'll get an error message, but if you specify only a single word for a directory, there's an amusing edge case with Docker Desktop, where this gets used as a name, and the mount is managed inside the hidden docker desktop container, which can cause confusing side-effects.

* Use the same collation as xwiki does by default

* Also update the compose files

* Typo

In order to use secrets for mysql root password the parameter needs to MYSQL_ROOT_PASSWORD_FILE

Fix broken link

* Typo2

* Typo

* Improved a bit the PR from Ashish

* Fix README to follow guidelines from Dockerhub + small improvements

* 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