Mounting /var/www/app will over-write the default app with an empty folder. The app has been copied to /tmp/app so you can use docker cp to copy the default app to your volume mount. node_modules has a problem copying in windows, you me need to delete it from the /tmp/app before initiating the docker cp.
entrypoint by default checks for /var/www/app/node_modules and will run npm install if it doesn't find it. Then it checks for cmd if no arguments are passed, it defaults to running npm start otherwise it passes all arguments to exec