Fix mastodon:setup to take dotenv/docker-compose differences into account (mastodon#16896)

In order to work around mastodon#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

Author: ClearlyClaire

lib/tasks/mastodon.rake

@@ -350,11 +350,11 @@ namespace :mastodon do
           end
         end.join("\n")
 
-        generated_header = "# Generated with mastodon:setup on #{Time.now.utc}\n\n".dup
+        generated_header = "# Generated with mastodon:setup on #{Time.now.utc}\n\n"
 
         if incompatible_syntax
-          generated_header << "# Some variables in this file will be interpreted differently whether you are\n"
-          generated_header << "# using docker-compose or not.\n\n"
+          generated_header << "Some variables in this file will be interpreted differently whether you are\n"
+          generated_header << "using docker-compose or not.\n\n"
         end
 
         File.write(Rails.root.join('.env.production'), "#{generated_header}#{env_contents}\n")