I am following the official docker tutorial:
<https://docs.docker.com/get-started/part2/#build-the-app>

I can successfully build the Docker image (after creating the Dockerfile, app.py and requirements.txt) and see it:

```
docker build -t friendlyhello .
docker ps -a

```

However, it quits immediately when running

```
docker run -p 4000:80 friendlyhello

```

I cannot find the way to find why it did not work

1) "docker ps -a" - says the container exited
2) docker logs "container name" returns no information about logs
3) I can attach the shell to it: 

```
docker run  -p 4000:80 friendlyhello /bin/sh

```

but I did not manage to find (grep) any logging information there (in /var/log)
4) attaching foreground and detached mode with -t and -d did not help

What else could I do?