When you say ./launcher destroy app it means that launcher will operate on the site defined in the app.yml file. In order to parse that app.yml file and construct all the logic in it, we need some software. And since we can’t feasibly control which software exists on the thousands of different hosts running Discourse, we use the same base image we use for Discourse to parse and interpret what’s on that app.yml file, hence we need to download it.
If you can’t afford the space, you can always to down an abstraction level to the docker commands directly. In this example, the equivalent would be docker stop app && docker rm app.