/var/www/html/maintenance/update.php needs to be executed after a fresh `docker-compose up`

[rht]

Either document it that people must manually do docker exec pkc_mediawiki_1 php /var/www/html/maintenance/update.php after their first docker-compose up -d, or somehow specify the script execution in docker-compose.yml.

Relevant issue: docker/compose#1809 (which is a duplicate of many issues).

I think for now it is better to just document the step.