A container is a program running in a space of its own, with everything it needs already packed in an image. It lives as long as its main process lives: if the program ends or fails, the container stops. Almost everything that goes wrong at first is settled with two commands, docker ps -a and docker logs. This article assumes Docker is installed on a VPS: here is how to install it.
Starting a test web server
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Start the container: docker run -d --name test -p 8080:80 nginxThe -d keeps it in the background, --name gives it a name, and -p 8080:80 connects the server’s port 8080 to the container’s port 80. The nginx image is downloaded the first time.
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See whether it is up: docker psIt shows only those running. With docker ps -a it shows the stopped ones too.
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Ask for the page, on the server itself: curl http://localhost:8080. You get nginx’s welcome HTML.
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Read the logs: docker logs test, or add -f to follow live, and --tail 100 for just the last lines.
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Stop and delete: docker stop test
docker rm testStopping does not delete. While the container exists, the name test stays taken.
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When it will not start: what the symptom means
| What you see |
What it usually is |
What to do |
| Status “Exited (0)” |
The program ended normally. A container with nothing to do ends at once. |
If you expected a service, the image or command is not a service. Read the image’s page. |
| Status “Exited (1)” |
The program failed on start: a missing file, an unset variable, a configuration error. |
docker logs name: the last line usually says why. |
| Status “Exited (137)” |
The container was killed. Often it is the server running out of memory. |
free -h, and see whether the VPS has free memory. |
| “Restarting” |
It fails and starts again, in a loop, because of the restart policy. |
The logs, as with Exited (1). Stop it first: docker stop name. |
| “port is already allocated” |
The port you asked for is already used by something else. |
sudo ss -tlnp shows who has it. Pick another port on the left-hand side. |
| “pull access denied” or “not found” |
A misspelt image name, or a private image with no login. |
Check the name at hub.docker.com. |
“permission denied” on docker.sock |
Your user is not in the docker group. |
Use sudo, or see the end of the installation article. |
To look inside a container that is running: docker exec -it test shIt opens a command line in there. Leave with exit. To see the full configuration (ports, volumes, variables): docker inspect test.
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What is inside a container disappears with it. If you delete the container, the files the program created in there go too, unless you connected a folder or a volume. Before putting important data in a container, read Docker Compose: an application with its database.
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Publishing a port opens it to the world. With -p 8080:80 anyone can reach it, and Docker steps around the ufw rules. For tests on the server only, use -p 127.0.0.1:8080:80. See ports and firewall on a VPS.
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Server not answering, or SSH no longer getting in? That is ours: tell us what you see and when it began. The application inside the container is yours, and the guide above helps you find what makes it fail.
Open a support ticket
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