How do your dependencies get installed? What happens if you depend on something in your package.json that isn't on the public npm repo? What happens if you need to compile some native code as part of one of your dependencies?

Do you need to depend on anything provided by the OS? Timezone data? Maybe you use sqlite as a storage backend and want to ship your database file as well. How do you include configuration data?

Docker drastically simplifies packaging and deployment, and gives you a ton of flexibility in where and how you run your code.