You can do something like this inside the container to add a plugin to an existing container:
cd /var/www/discourse/plugins
git clone my-plugin-url
rake db:migrate assets:precompile
sv restart unicorn
But the problems of keeping the plugins in the container and in the app.yml for when you rebuild next are there.
I’ve been burned a number of times doing a ./launcher destroy app;./launcher start app to apply new ENV variables from app.yml only to find that the “new” container is behind the version that the database is. It’s even worse if someone has upgrade only some plugins in the container that worked with the version of Discourse that was in the old container, but not with the one that you get when you rebuild. . .