add comments and handle default app_executable and app_image

[magland]

@luiztauffer

I added some code comments.

You will see there is now the following behavior

if app_image is set, then app_executable default to /app/main.py

if app_image is not set, then app_executable needs to be set to the local file system path to main.py. In this case we run the processor outside a container. Useful for development.