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Docker

Misc commands

Memory and CPU usage

docker stats

Like Linux top

Delete all stopped containers

docker container prune

History

docker history <Image Name>

Show the infor on each layer in the specified Docker image.

Dangling images

i.e. Images with Name "".

docker rmi $(docker images --filter "dangling=true" -q --no-trunc)

To avoid: Add --rm to docker build.

Network - misc

List available networks

docker network ls

Specify a network during docker run

For example:

docker run .... --network="host"

Publish Port

docker run -p 8080:80 <image>

The first port is Docker host, the second port is Docker container.

Add an /etc/hosts entry

docker run --add-host=example.com:10.0.0.1 --rm -it alpine grep example.com /etc/hosts

Volumes

https://www.baeldung.com/ops/docker-volumes

Mount Windows volumes

docker run -v //c/:/c --rm -it alpine ls /c

PostgreSQL specific:

docker run --rm --network=host -v "//c/:/c" postgres pg_dump --format=custom -d db_name --verbose -f "/c/tmp/dump.backup"

Busybox ("The Swiss Army knife of Embedded Linux"):

docker run -v //c/:/c --rm -it busybox ash # Shell

Mount host pwd

docker run --rm -it -v $(pwd):/host_pwd busybox

Small

Inspect

docker inspect `docker ps | grep <container name> | cut -f1 -d" "` | more

Run, interactive command line, delete upon exit

docker run --rm -it alpine

Run, some command(s) first, then interactive command line

docker run --rm -it -v $(pwd):/host_pwd \
  ubuntu /bin/bash \
  -c 'cd /host_pwd; exec bash'

Run, start ssh-agent first, then interactive command line

docker run --rm -it -v $(pwd):/host_pwd \
  jenkins/ssh-agent /bin/bash \
  -c 'eval $(ssh-agent) > /dev/null; echo $SSH_AUTH_SOCK; exec bash'

Run, with name

docker run --name alpine-today --rm -it alpine

Shell access

docker exec -it <mycontainer> bash

Run indefinitely and detached

docker run -d alpine tail -f /dev/null

Using a Docker environment variable from a command required a shell.

Works: docker exec -it ccc sh -c 'echo $MY_ENV_VAR'

Doesn't work (for various reasons): docker exec -it ccc echo $MY_ENV_VAR

Get the result of a 'docker exec' command

docker exec -t -i my-container sh -c 'my-command; exit $?'

Check that the docker daemon is running, and that you have permission to use it.

docker info > /dev/null 2>&1
if [ $? != "0" ]; then
   echo ERROR: Docker daemon is not running, and/or you\'re not in the \'docker\' group.>&2
   exit 1
fi

Security

Containers are not completely isolated from their hosts (unlike VMs.) Their atually run directly on the host, but just in their own namespace.

Dockerfile

ENTRYPOINT

Like CMD, except that the docker run parameters (if present) get appended to it. If not present, then the CMD contents are used.

Overridden by docker's --entrypoint parameter.

Misc useful images

ssh

docker run -it --rm kroniak/ssh-client ssh root@example.com

Architecture

containerd

High-level industry-standard container runtime. Implements Open Container Initiative (OCI)-compliant runtimes, between OS and Docker.

A standalone CNCF project.

Good post: https://www.docker.com/blog/containerd-vs-docker/

Kernel

uname -a from a Docker container shows the kernel information of the Docker host, not the container itself, because Docker containers share the host's kernel.

Consider: /etc/os-release for OS details, which will match the Dockerfile/Containerfile origin.

Docker Hub

Digests

Index digest vs Manifest digest

e.g. https://hub.docker.com/layers/bitwarden/self-host/2024.7.2-beta/images/sha256-746f057421a8fec924662325395d7de53d524799ee56c9dd3078336ff8e91c20?context=explore

Index digest: sha256:73ce...

Manifest digests:

  • linux/amd64: sha256:746f...
  • linux/arm74: sha256:20d3...
docker pull bitwarden/self-host:2024.7.2-beta
...
docker image ls --digests  bitwarden/self-host
REPOSITORY            TAG             DIGEST           IMAGE ID       CREATED       SIZE
bitwarden/self-host   2024.7.2-beta   sha256:20d3...   b5a485c3c35a   11 days ago   1.01GB
bitwarden/self-host   2024.7.2-beta   sha256:73ce...   b5a485c3c35a   11 days ago   1.01GB