We have noticed that our documentation is missing a step for users using the Google Cloud ssh console (as it seems to be your case). As you well say, because the google console starts with a user different than
bitnami, it generates a permissions error. The correct steps are:
$ cd /opt/bitnami/apps/discourse/htdocs
$ sudo su bitnami
$ RAILS_ENV=production bundle exec rake plugin:install repo=PLUGIN_REPO_URL
$ RAILS_ENV=production bundle exec rake assets:precompile
Your solution should work without any problem. We will fix our documentation soon.
Carlos R. Hernández