Then nothing is listening on port 80?
If docker was listening on :80, which any successful install would achieve (and require for nginx inside the container to see anything) you would see:
COMMAND PID USER FD TYPE DEVICE SIZE/OFF NODE NAME
docker-pr 890 root 4u IPv6 961922 0t0 TCP *:http (LISTEN)
If nginx isn’t installed outside the container, and Docker isn’t listening on that port, then the only other way to be seeing the nginx welcome page is a network misconfiguration.