Fix mastodon:setup to take dotenv/docker-compose differences into account (#16896)
In order to work around https://github.com/mastodon/mastodon/issues/16895,
add a warning to .env.production.sample, and change the mastodon:setup rake
task to:
- output a warning if a variable will be interpreted differently by dotenv
and docker-compose
- ensure the printed config is compatible with docker-compose