Right… It needs to match one of the keys in the [processes] stanza. This is connecting your Django server to the Fly Proxy, and thence to the public Internet.
Yes, the values in quotes under [processes] override the CMD from the Dockerfile, basically.
https://fly.io/docs/reference/configuration/#the-processes-section
Probably you also want "supercronic /code/crontab" for the cron line, since your Dockerfile copied crontab into /code/ instead of /app/ (due to WORKDIR).