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Docker for Beginners: Run an App Without Fighting the Host
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Docker for Beginners: Run an App Without Fighting the Host

Check Yourself: Your First Containers

Five scenario questions on ports, exit codes, logs, exec and cp. If you miss more than one, reread the lessons in this section.

5 min DockerTesting and DebuggingDeployment and Containers Beginner
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1. You run docker run -d --name hello-web -p 8080:80 nginx:1.30-alpine. What does 8080:80 mean?
Docker waits 8080 milliseconds before opening port 80. Port 8080 on your computer goes to port 80 inside the container. The container uses ports 8080 to 80, a range of 72 ports. Port 80 on your computer goes to port 8080 inside the container.
2. You try to start a second container with the same name and get an error that the name is already in use. The first container is stopped. What is the cleanest fix?
Delete the nginx image. Restart Docker Desktop, because stopped containers lose their names. Add the -d flag so Docker ignores the conflict. Remove the old container, or pick another name for the new one.
3. docker run -p 8080:80 fails with "port is already allocated" for port 8080. What is the most likely cause?
Port 80 inside the container is already taken by a previous container. The container name is invalid. The nginx image is missing, so Docker cannot open any port. Another container or program on your computer is already using port 8080.
4. You write a file inside a running container with docker exec, then remove the container with docker rm -f and start a new one from the same image. Is the file there?
No. The file lived in the old container's writable layer, which was deleted with it. Yes. Docker saves everything written in a container when it is removed. Only if you wrote the file as an administrator. Yes, but only if the container was started with -d.
5. A container shows Exited (3) in docker ps -a. Where do you look first for the reason?
Nowhere. Exit codes are random. The size of the image. docker logs with the container name, and then the exit code. The Docker Desktop settings page.

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Five scenario questions on ports, exit codes, logs, exec and cp. If you miss more than one, reread the lessons in this section.

Duration
5 min
Last update
08/04/2026
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