To install a plugin, open a Web terminal and run the following commands:
cd /app/code/plugins
git clone <plugin-repo>
chown -R cloudron:cloudron <plugin-dir>
cd /app/code
gosu cloudron:cloudron bundle exec rake plugin:install_all_gems
gosu cloudron:cloudron bundle exec rake db:migrate
gosu cloudron:cloudron bundle exec rake assets:precompile

Restart the application to double check everything works.

If the plugin modifies the posts in some way, you might want to rebake posts.
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To rebuild all posts (for example, to apply formatting provided by a newly installed plugin to old posts), open a Web terminal and run the following command:
cd /app/code
gosu cloudron:cloudron bundle exec rake posts:rebake

Warning:
Install with care. Use caution when installing plugins since they might break your installation. The list of official plugins can be found here.