Debug Docker that quits immediately?

Question

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?

Answer 1

Note: a docker exec on an exited (stopped) container should not be possible (see moby issue 30361)

docker logs and docker inspect on a stopped container should still be possible, but docker exec indeed not.

You should see

Error response from daemon: Container a21... is not running

So a docker inspect of the image you are running should reveal the entrypoint and cmd, as in this answer.
The normal behavior is the one described in this answer.