It looks like they way your instructions are written you are expecting a node_modules folder to be present in the local directory so that it gets packaged into the container image. As it stands if you clone the repo and then build the image it will not run because npm install has not yet been run. I would suggest either:

1. Update the instructions to have the user run npm install before they docker build so that node_modules gets created. This would obviously also require the user to have Node.js installed on their local workstation, so this leads me to option 2.

2. Add a RUN npm install instruction to your Dockerfile

Unless it's in there and I just missed it - but two people on my team experienced an ERROR 503 when running the app.

Oh, and, this is a great tutorial. Well done!