Move npm install above the source copy in Dockerfile
Move npm install above the source copy in Dockerfile

This allows us to bypass npm installing every time we adjust the app's source code.

z123/build-a-saas-app-with-flask,nickjj/build-a-saas-app-with-flask,nickjj/build-a-saas-app-with-flask,z123/build-a-saas-app-with-flask,nickjj/build-a-saas-app-with-flask,nickjj/build-a-saas-app-with-flask,z123/build-a-saas-app-with-flask