Solr re-indexes many documents after restarting on PostgreSQL

The sorting issue is caused by the locale of the database. This locale isn't set in XWiki, it needs to be specified when creating the database. I've updated the instructions on https://www.xwiki.org/xwiki/bin/view/Documentation/AdminGuide/Installation/InstallationWAR/InstallationPostgreSQL/ to create databases with the correct encoding.

Using the locale "C" or "C.UTF-8" works. "C.UTF-8" seems better in the sense that it also supports uppercase/lowercase transformations for non-ASCII characters.

Unfortunately, the locale cannot be changed after the database has been created. Therefore, the only fix for existing installations is to dump and re-create the database.

Things to check and do:

1. Can/do we set the correct locale for integration tests, also to verify that everything works correctly with the recommended locale?
2. Can we make sure that the Debian package creates the database with the correct locale?
3. Update the instructions in the Docker README.