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docker.md

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Install docker

yum install docker

Get docker running

systemctl enable docker
systemctl start docker

Test docker

docker run hello-world

List downloaded images (these take disk space)

docker image ls
# or just
docker images

List containers (including stopped containers)

docker container ls --all
# or just
docker ps -a

Add --size to see how much space each container is taking.

Remove a container

docker container rm <container_id>

Remove an image

docker image rm <image_id>

Build a Docker image

After creating a Dockerfile, you can build an image like this:

docker build --tag=myapp .
# or
docker build --tag=myapp:v0.0.1 .

If you have a problem resolving DNS

For example:

Temporary failure resolving 'security.debian.org'

Quick fix:

docker build --network=host ...
docker run --network=host ...

Permanent fix:

cat >> /etc/docker/daemon.json << !
{
  "dns": ["1.1.1.1", "8.8.8.8"]
}
!

service docker restart

Source 1, Source 2

Clear unused images to reclaim disk space

Short way

Cleans up unused caches and stopped containers.

docker system prune -f

Long way

First (optional but recommended) remove any containers which are not running:

docker rm $(docker ps --filter "status=exited" -q --no-trunc)

Now clear intermediate images from partial builds (caches):

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

But this might be a better way to clean only intermediate images:

docker builder prune

Remove ALL docker containers and images

WARNING: This will destroy everything apart from running containers and their images

docker system prune --all

WARNING: This will destroy absolutely everything, including running containers!

docker stop $(docker ps -q)
docker kill $(docker ps -q)
docker container rm $(docker ps -a -q)
docker image rm -f $(docker images -a -q)

Actually it might not destroy all volumes. Check:

du -sh /var/lib/docker/volumes

Start a named docker container

If you give your container a name when you start it, it will be easier to find it later.

docker run --detach --name <container_name> <image_name>

Start a docker container in interactive mode

If you want to interact with the process that the image starts up with.

docker run -it <image_name>

Enter a docker image to debug

docker run --rm -it <image_id> /bin/bash

In this case we have used --rm (optional) to automatically remove the container when it stops.

Note that the above may not work if the Docker image has specified an entrypoint. In such cases, we may need to override the entrypoint like so:

docker run --rm -it --entrypoint /bin/bash <image_id>

Enter a docker container to debug

If your container has stopped, you must first restart it.

docker restart <container_name>

Now you can open a shell inside the container, to debug it:

docker exec -it <container_name> /bin/bash

Some images set a default user. If you want to enter the container as the root user, add -u root

Install some useful debugging tools

# For Alpine Linux
apk add bash procps findutils bind-tools nmap man vim curl

# For Debian
apt-get update && apt-get -y install procps findutils inetutils-ping dnsutils nmap man vim curl net-tools

alias l='ls -lartFh --color'

Stop a docker container

docker stop <container_name>

Copy files into or out of a docker container

docker cp <container_id>:/path /path
docker cp /path <container_id>:/path

This even works with containers which are not running!

We can also remove files:

docker rm -fv /unwanted/file

Copy files out of a docker image

We can even copy from an image without starting it.

docker create --name temp <image_name>

docker cp temp:/path /path

Working around the symlinks bug when copying

Docker can have trouble copying symlinks:

invalid symlink "/foo/bar" -> "../baz"

We can work around that by copying to stdout and then extracting:

docker cp <container_id>:/path - | tar x

# Or safer:
mkdir dest
docker cp <container_id>:/path - | tar -C dest -x

Export and import

We can also export an entire container (running or stopped):

docker export <container_name> > container.tar
docker export <container_name> | gzip -c > container.tgz

and import it again later as a new image:

docker import container.tgz <image_repository_name>

but note that in this case the image is not layered, so it will take the full size.

Other docker tools

  • nsenter - Easy way to start a shell inside a container (although docker exec has now taken its place). Includes docker-enter.

  • baseimage - A Docker image designed to run a full Linux system, rather than just one process. (syslog, cron, ssh, logrotate, ...)

See also