docs: clarify that a dockerfile is still needed
docs: clarify that a dockerfile is still needed

[ci skip]
jimjag/dokku,claytonbrown/dokku,claytonbrown/dokku,dokku/dokku,dokku/dokku,claytonbrown/dokku,jimjag/dokku,progrium/dokku,dokku/dokku,claytonbrown/dokku,progrium/dokku,claytonbrown/dokku,progrium/dokku,jimjag/dokku,progrium/dokku,dokku/dokku,claytonbrown/dokku,claytonbrown/dokku,jimjag/dokku,progrium/dokku,jimjag/dokku,dokku/dokku,jimjag/dokku,dokku/dokku,dokku/dokku,progrium/dokku,claytonbrown/dokku,progrium/dokku,progrium/dokku,jimjag/dokku,jimjag/dokku,dokku/dokku