docs: clarify usage of docker-based installation
docs: clarify usage of docker-based installation

ssh-key addition wasn't previously scoped out.

jimjag/dokku,claytonbrown/dokku,jimjag/dokku,dokku/dokku,jimjag/dokku,claytonbrown/dokku,progrium/dokku,progrium/dokku,jimjag/dokku,progrium/dokku,dokku/dokku,claytonbrown/dokku,jimjag/dokku,dokku/dokku,progrium/dokku,claytonbrown/dokku,claytonbrown/dokku,dokku/dokku,dokku/dokku,dokku/dokku,claytonbrown/dokku,progrium/dokku,progrium/dokku,claytonbrown/dokku,jimjag/dokku,dokku/dokku,progrium/dokku,jimjag/dokku,claytonbrown/dokku,dokku/dokku,jimjag/dokku,progrium/dokku