MySQL passwords not consistent

It looks to me the usage of MYSQL_PASSWORD and MYSQL_ROOT_PASSWORD is currently not consistent:

• scripts/install generates MYSQL_ROOT_PASSWORD but it does not actually write it to .env.
• docker-compose.yml app section uses MYSQL_PASSWORD not MYSQL_ROOT_PASSWORD

I propose to use the auto configuration env variables as described on https://github.com/nextcloud/docker#auto-configuration-via-environment-variables and https://github.com/nextcloud/docker#base-version---fpm and not use MYSQL_ROOT_PASSWORD in the app section:

• MYSQL_DATABASE nextcloud
• MYSQL_USER nextcloud
• MYSQL_PASSWORD provisioned by scripts/install
• MYSQL_HOST db

I will submit a MR for this and together with the other MR, I think the provision script will produce a running Nextcloud instance.