Change mastodon:setup to not call assets:precompile in docker (#13942)

It appears assets are built during image build, and they shouldn't need
to be rebuilt, since we now have reproducible builds.

lib/tasks/mastodon.rake

@@ -371,18 +371,20 @@ namespace :mastodon do
           end
         end
 
-        prompt.say "\n"
-        prompt.say 'The final step is compiling CSS/JS assets.'
-        prompt.say 'This may take a while and consume a lot of RAM.'
+        unless using_docker
+          prompt.say "\n"
+          prompt.say 'The final step is compiling CSS/JS assets.'
+          prompt.say 'This may take a while and consume a lot of RAM.'
 
-        if prompt.yes?('Compile the assets now?')
-          prompt.say 'Running `RAILS_ENV=production rails assets:precompile` ...'
-          prompt.say "\n\n"
+          if prompt.yes?('Compile the assets now?')
+            prompt.say 'Running `RAILS_ENV=production rails assets:precompile` ...'
+            prompt.say "\n\n"
 
-          if !system(env.transform_values(&:to_s).merge({ 'RAILS_ENV' => 'production' }), 'rails assets:precompile')
-            prompt.error 'That failed! Maybe you need swap space?'
-          else
-            prompt.say 'Done!'
+            if !system(env.transform_values(&:to_s).merge({ 'RAILS_ENV' => 'production' }), 'rails assets:precompile')
+              prompt.error 'That failed! Maybe you need swap space?'
+            else
+              prompt.say 'Done!'
+            end
           end
         end