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

.env.production.sample

@@ -4,6 +4,12 @@
 # not demonstrate all available configuration options. Please look at
 # https://docs.joinmastodon.org/admin/config/ for the full documentation.
 
+# Note that this file accepts slightly different syntax depending on whether
+# you are using `docker-compose` or not. In particular, if you use
+# `docker-compose`, the value of each declared variable will be taken verbatim,
+# including surrounding quotes.
+# See: https://github.com/mastodon/mastodon/issues/16895
+
 # Federation
 # ----------
 # This identifies your server and cannot be changed safely later

lib/tasks/mastodon.rake

@@ -333,8 +333,12 @@ namespace :mastodon do
       prompt.say 'This configuration will be written to .env.production'
 
       if prompt.yes?('Save configuration?')
+        incompatible_syntax = false
+
         env_contents = env.each_pair.map do |key, value|
           if value.is_a?(String) && value =~ /[\s\#\\"]/
+            incompatible_syntax = true
+
             if value =~ /[']/
               value = value.to_s.gsub(/[\\"\$]/) { |x| "\\#{x}" }
               "#{key}=\"#{value}\""
@@ -346,12 +350,19 @@ namespace :mastodon do
           end
         end.join("\n")
 
-        File.write(Rails.root.join('.env.production'), "# Generated with mastodon:setup on #{Time.now.utc}\n\n" + env_contents + "\n")
+        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"
+        end
+
+        File.write(Rails.root.join('.env.production'), "#{generated_header}#{env_contents}\n")
 
         if using_docker
           prompt.ok 'Below is your configuration, save it to an .env.production file outside Docker:'
           prompt.say "\n"
-          prompt.say File.read(Rails.root.join('.env.production'))
+          prompt.say "#{generated_header}#{env.each_pair.map { |key, value| "#{key}=#{value}" }.join("\n")}"
           prompt.say "\n"
           prompt.ok 'It is also saved within this container so you can proceed with this wizard.'
         end