The last step (rebuild web_only) fails:
Bundle complete! 110 Gemfile dependencies, 201 gems now installed.
Gems in the group development were not installed.
Bundled gems are installed into `./vendor/bundle`
I, [2018-08-31T00:50:57.518677 #13] INFO -- : > cd /var/www/discourse && su discourse -c 'bundle exec rake db:migrate'
rake aborted!
Gem::LoadError: can't activate public_suffix-2.0.5, already activated public_suffix-3.0.2
/var/www/discourse/lib/plugin_gem.rb:18:in `load'
/var/www/discourse/lib/plugin/instance.rb:501:in `gem'
/var/www/discourse/plugins/discourse-sync-to-dropbox/plugin.rb:7:in `activate!'
/var/www/discourse/lib/plugin/instance.rb:431:in `instance_eval'
/var/www/discourse/lib/plugin/instance.rb:431:in `activate!'
lib/discourse.rb:164:in `block in activate_plugins!'
lib/discourse.rb:161:in `each'
lib/discourse.rb:161:in `activate_plugins!'
/var/www/discourse/config/application.rb:223:in `<class:Application>'
/var/www/discourse/config/application.rb:39:in `<module:Discourse>'
/var/www/discourse/config/application.rb:38:in `<top (required)>'
/var/www/discourse/Rakefile:5:in `require'
/var/www/discourse/Rakefile:5:in `<top (required)>'
/var/www/discourse/vendor/bundle/ruby/2.5.0/gems/rake-12.3.1/exe/rake:27:in `<top (required)>'
/usr/local/bin/bundle:23:in `load'
/usr/local/bin/bundle:23:in `<main>'
(See full trace by running task with --trace)
I, [2018-08-31T00:51:18.343389 #13] INFO -- : gem install geocoder -v 1.4.4 -i /var/www/discourse/plugins/discourse-locations/gems/2.5.1 --no-document --ignore-dependencies
Successfully installed geocoder-1.4.4
1 gem installed
gem install public_suffix -v 2.0.5 -i /var/www/discourse/plugins/discourse-sync-to-dropbox/gems/2.5.1 --no-document --ignore-dependencies
Successfully installed public_suffix-2.0.5
1 gem installed
FAILED
--------------------
Pups::ExecError: cd /var/www/discourse && su discourse -c 'bundle exec rake db:migrate' failed with return #<Process::Status: pid 507 exit 1>
Location of failure: /pups/lib/pups/exec_command.rb:112:in `spawn'
exec failed with the params {"cd"=>"$home", "hook"=>"bundle_exec", "cmd"=>["su discourse -c 'bundle install --deployment --verbose --without test --without development --retry 3 --jobs 4'", "su discourse -c 'bundle exec rake db:migrate'", "su discourse -c 'bundle exec rake assets:precompile'"]}
36b568dc13935b0491b99afd4763776899dad8d57a1df149633b9c7c02fd3c0b
** FAILED TO BOOTSTRAP ** please scroll up and look for earlier error messages, there may be more than one
Trying without the dropbox plugin now…