If bind mounts are specified like this, it doesn’t matter what gets baked into the container’s /src during build time. It will be over-written by whatever is in the host filesystem ./app at runtime.
The example tutorial doesn’t say anything about “NFL Crime Stats” ( https://github.com/getcarina/examples/blob/master/python-web-app/app/templates/index.html is the main page), so I’m guessing you’ve actually deviated from the tutorial material and are maybe trying to run your own app? You will need to ensure that the container can access the img directory correctly.
It’s hard to say exactly what’s wrong without more information or a reproducible example.