In case of despair…

If you get stuck, try the following from a shell in your host OS:

docker stop hey-spark
docker start hey-spark
docker exec hey-spark sh -c "python stream.py &"

Check the logs for obvious error notifications or warnings:

docker logs hey-spark

Check state of files/directories/etc. inside the container:

docker exec -it hey-spark /bin/bash

E.g., remove incomplete state from a previous failed attempt:

rm -rf /opt/hadoop/share/runedata /tmp/checkpoint*