Cannot run app into container successfully

[darribas]

I'm really curious to try this one out inside a container to play "safely". I've created a simple Dockerfile (very heavy, can slim down afterwards), and try to run both Flask and npm out of a JupyterLab session:

The above returns the following error:

Any idea what can be going on? I have zero experience with JS, so this might be horribly trivial... Thanks in advance!