See https://www.dokuwiki.org/install:unused_files for more informations

As explained on [this page](https://www.dokuwiki.org/install:upgrade), as the Dokuwiki files (PHP...) and user pages are "mixed" in the same directory, it is better to get the new Dokuwiki version in a separate directory, and then to copy the files.

This way, we won't have conflicts between old volumes and new files in the image. It is safer.

Update pica-dokuwiki/Dockerfile, pica-dokuwiki/docker-compose.yml, pica-dokuwiki/entrypoint.sh, pica-dokuwiki/nginx.conf, pica-dokuwiki/run.sh files