Docker Cheatsheet

Images

Building images

$ docker build -t myimage:0.001 .

Use a different Dockerfile:

$ docker build -t myimage:0.001 -f Dockerfile.myimage .

Skipping cache:

$ docker build --no-cache -t myimage:0.001 .

Prune images

Delete all images without an associated container:

$ docker image prune

Inspect images

In order to inspect the images, you may run them as container with a shell command - usually bash but sh for Alpine based ones:

$ docker run -it myimage:0.001 bash

Containers

Run from image

$ docker run --name sympa -d -i sympa:6.2.48

Explanation:

-d

detached

-i

image tag

--name

name of the container (optional)

You can instruct Docker to automatically remove the container when it is terminated by adding the --rm flag:

$ docker run --rm --name sympa -d -i sympa:6.2.48

Please note that this also removes the logs for the container.

Environment variables can be passed to the container with the --env flag:

$ docker run --env SYMPA_DB_USER=sympa --env SYMPA_DB_PASSWORD=sympa --name sympa -d -i sympa:6.2.48

Remove and prune

Remove container

docker rm sympa

Please note that the corresponding image isn't affected.

Prune containers

Remove all stopped containers:

$ docker container prune

Remove all stopped containers older than a day:

$ docker container prune --filter "until=24h"

Shell on a running container

$ docker exec -it 593d9b3219c6 bash

You can also pass environment variables to the command executed in the Docker container:

$ docker exec -e SPACEWALK_API_USER=admin -e SPACEWALK_API_PASS=nevairbe -it spacewalk /bin/setup-spacecmd.sh

Note: the flags for the environment preceed the flag for the container.

Copy files from/to container

From:

$ docker cp wp-site:/var/www/html/wp-config.php .

To:

$ docker cp  ./setup-spacecmd.sh spacewalk2:/bin

Logs

To see the log messages for the container nginx:

$ docker logs nginx

However, this shows only the messages without any further information.

You can add the timestamp with the --timestamps commandline option:

$ docker logs --timestamps nginx

Show the latest 30 entries:

$ docker logs --tail 30 nginx

Follow new entries:

$ docker logs --tail 0 nginx --follow

Container Information

IP address

To get the IP address of the container perldancer:

$ docker inspect -f '{{range .NetworkSettings.Networks}}{{.IPAddress}}{{end}}' perldancer

Determine whether a container is running properly

This checks whether the container with the name sympa is active and running, e.g. not restarting because of a problem with the command it executes on startup.

$ docker ps -q --filter=name='/sympa$' --filter=status=running

Startup problems

• Missing environment variables, e.g. MYSQL_ROOT_PASSWORD for mysql container

Compose

Run containers and detach:

$ docker-compose up -d

Proxy

Configure proxy in /etc/docker/daemon.json:

{
  "proxies": {
    "http-proxy": "http://10.11.12.13:3128/",
    "https-proxy": "http://10.11.12.13:3128/",
    "no-proxy": ".example.org"
  }
}