Automatically build SOLR docker image with compose

I've opend a pull request: https://github.com/xwiki-contrib/docker-xwiki/pull/14

• Add Dockerfile for a standalone SOLR container, see:
  https://www.xwiki.org/xwiki/bin/view/Documentation/AdminGuide/Performances/#HStandaloneSolr
  https://extensions.xwiki.org/xwiki/bin/view/Extension/Solr%20Search%20API#HSetuparemoteSolrserver
  The image automatically downloads the corresponding

  xwiki-platform-search-solr-server-data.jar

• Add demo compose file docker-compose-postgres-index.yml , to use the external SOLR just execute:

docker-compose -f docker-compose-postgres-index.yml up -d