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Docker Cheatsheet

Last Updated: 2024-04-09

Docker Images and Containers

# Lists images stored in the docker daemon's data root, by default /var/lib/docker.
$ docker image ls

# List all containers
$ docker ps -a

# List running containers including CPU/memory size
$ docker ps -s

# Specify format
$ docker ps -a --format "{{.ID}},{{.Names}},{{.Status}},{{.Image}},{{.Ports}}"

# Go inside the container; `-i` for `interactive, `t` for `terminal.
$ docker exec -it <container> bash

# Execute
$ docker exec -i ... # interactive mode
$ docker exec -d ... # detached mode

# Start a Ubuntu 20.04
$ docker run -it --entrypoint "/bin/bash" ubuntu:20.04

# Export a container’s filesystem as a tar archive
$ docker export --output="latest.tar" <container>

# port forwarding
$ docker run -p 8080:8080 myserver

# kill all containers
$ docker kill $(docker ps -q)

# Remove all containers in exited status
$ docker rm $(docker ps -a -f status=exited -q)

# Check port mapping
$ docker port CONTAINER_NAME
6443/tcp -> 127.0.0.1:34305

# Remove unused data (images, containters, networks, build cache, etc)
$ docker system prune

Docker Info and Daemon

Docker daemon communicates with the Linux kernel in order to create containers. docker CLI -> dockerd -> kernel.

# Run Docker daemon in the foreground
$ dockerd

# Run Docker daemon as a systemd service
$ sudo systemctl start docker

# Check if dockerd is running
$ sudo systemctl status docker

# Check docker status
$ curl -s --unix-socket /var/run/docker.sock http/_ping
OK%

# Check docker info (these 2 are the same)
$ docker info
$ docker system info

# Display the SBOM (Software Bill Of Materials) of any Docker image. This feature outputs the SBOM in a table or can be exported into SPDX and CycloneDX formats.
$ docker sbom

# Check disk usage
$ docker system df
$ docker system df -v  # v for verbose

Docker Registries

# Create a local registry
$ docker run -d -p 5000:5000 --restart=always --name registry registry:2

# 5000 is used inside the container (hardcoded in Dockerfile);
# if 5000 is already used on local machine, change it to another port like 5001
$ docker run -d -p 5001:5000 --restart=always --name registry registry:2

# Check if registry is running
$ docker ps

# If needs login
$ docker login $REGISTRY

# Pull from docker.io/library/ubuntu:latest
$ docker pull ubuntu

# Tag and Push to local registry
$ docker tag ubuntu:22.04 $REGISTRY/demo/ubuntu:22.04
$ docker push $REGISTRY/demo/ubuntu:22.04

# Pull from local registry
$ docker pull $REGISTRY/library/ubuntu:22.04

Check registry:

$ curl -X GET http://myregistry:5000/v2/_catalog
{"repositories":["myimage"]}

$ curl -X GET http://myregistry:5000/v2/mymage/tags/list
{"name":"myimage","tags":["latest"]}

# with user and password
$ curl -X GET -u <user>:<pass> https://myregistry:5000/v2/_catalog

# with cert
$ curl --cacert domain.crt https://myregistry:5000/v2/_catalog

Docker image name = repository + ":" + tag

Source Code

The Git repo of the official Docker image for distribution. https://github.com/docker/distribution-library-image/tree/master

https://github.com/distribution/distribution

Download and run registry locally:

wget https://github.com/distribution/distribution/releases/download/v2.8.2/registry_2.8.2_linux_amd64.tar.gz

tar xvzf registry_2.8.2_linux_amd64.tar.gz

./registry --version
./registry github.com/docker/distribution 2.8.2

Notice github.com/docker/distribution instead of github.com/distribution/distribution

Registry Mirror

Docker registry mirror: add registry-mirrors to the /etc/docker/daemon.json configuration file.

{
  ...
  "registry-mirrors": ["https://mirror.gcr.io"]
}

Now docker pull will pull from the https://mirror.gcr.io instead of docker.io.

Side note: containerd registry mirror can be configured in /etc/containerd/config.toml

The official image: registry:2

Docker Networking

# List networks
$ docker network ls

# Create network
$ docker network create

# Remove network
$ docker network rm

# Connect container to network
$ docker network connect NETWORK_NAME CONTAINER_NAME

# Verify the `CONTAINER_NAME` is added to the `Containers` field of the network:
$ docker inspect NETWORK_NAME --type network

Check by docker network ls

Name Driver Note
bridge bridge Docker's default
kind bridge kind's default
host host no network isolation
none null

DRIVER:

  • null: the container does not have external network interfaces, only a local loopback interface.
  • host: the container is attached to the host's network, the configs inside the container matches the configs outside the container.
  • bridge: containers connected to the same bridge network can communicate; containers on different bridge networks cannot communicate directly with each other.

To get more details:

$ docker network inspect bridge

Containers Networking Options

  • container to container on the same node (the same Docker daemon host): through a bridge network
  • container to container on different nodes: through a overlay network
  • container to outside world: --publish or -p <hostport>:<containerport>

DNS

Inherit /etc/resolv.conf on the host, but can override per-container in docker run / docker create using the flags: --dns, --dns-search, --dns-opt, --hostname.

Isolation

On Linux, Docker manipulates iptables rules to provide network isolation.

credHelper

credHelpers can be set in ~/.docker/config.json

"credHelpers": {
    "gcr.io": "gcloud"
}

It means any image pull from gcr.io will use the binary docker-credential-gcloud to get the username and secret. (The binary = docker-credential- + suffix.)

$ echo gcr.io | docker-credential-gcloud get

To login:

$ docker login 10.200.0.1 -u admin -p ${REGISTRY_PASSWORD}

Configurations and Files

Docker Root Dir (used by the docker daemon to keep track of the state).

  • /var/lib/docker/, e.g.
    • /var/lib/docker/overlay2: image and container layers.
    • /var/lib/docker/volumes

Server ("Rootful"):

  • dockerd (daemon) config: /etc/docker/daemon.json
  • Socket: /var/run/docker.sock

Server (Rootless Mode):

  • dockerd (daemon) config: ~/.config/docker
  • Socket: $XDG_RUNTIME_DIR/docker.sock
    • $XDG_RUNTIME_DIR is typically set to /run/user/$UID
  • Data dir: ~/.local/share/docker by default

Client:

  • ~/.docker/config.json is for the docker CLI.

Registry:

  • config: /etc/registry/config.yaml
  • storage: /var/lib/registry

Dockerfile

CMD vs ENTRYPOINT

Both CMD and ENTRYPOINT instructions define what command gets executed when running a container. There are few rules that describe their co-operation.

  • Dockerfile should specify at least one of CMD or ENTRYPOINT commands.
  • ENTRYPOINT should be defined when using the container as an executable.
  • CMD should be used as a way of defining default arguments for an ENTRYPOINT command or for executing an ad-hoc command in a container.
  • CMD will be overridden when running the container with alternative arguments.

Example: Containerize a Node app

Use this Dockerfile template:

# node
FROM node:12-slim

# the path inside the container
WORKDIR /usr/src/app

# copy the package.json and package-lock.json, and install dependencies
COPY package*.json ./
RUN npm install

# copy all the source code
COPY . .

# port
EXPOSE 8080

# run the command inside the container
CMD [ "node", "app.js" ]

And add a .dockerignore file:

Dockerfile
.dockerignore
node_modules
npm-debug.log

FAQ

What is Docker?

When we talk about Docker, usually we are referring to Docker Engine, which consists of

  1. the Docker daemon (dockerd).
  2. a REST API that specifies interfaces for interacting with the daemon.
  3. a command line interface (CLI) client (docker) that talks to the daemon (through the REST API wrapper), e.g. docker run <image>, docker image ls.

Because Docker operates at the OS level, it can still be run inside a VM.

2 most important APIs: Images and Container APIs.

What's the virtual size?

virtual = image size + container layers size.

Docker Compose vs Docker Stack vs Swarm

  • docker compose: a tool for defining and running multi-container Docker applications; v1 (the separate binary docker-compose was deprecated), v2 is a subcommand compose in the docker CLI.
  • docker stack: Manage Swarm stacks.
  • both works with docker-compose.yml, however docker stack only works with version 3.
  • Swarm: a cluster of Docker daemons. Now swarm is also a subcommand of docker CLI. Compose: multiple containers on the same host (daemon).

import vs load

  • import is used with the tarball which are created with docker export.
  • load is used with the tarball which are created with docker save.

Docker for Mac

The Docker for Mac application does not use docker-machine to provision that VM; but rather creates and manages it directly.

Rootless Mode

Allows running the dockerd and containers as a non-root user to mitigate potential vulnerabilities in the daemon and the container runtime.

Rootless mode executes the Docker daemon and containers inside a user namespace.

Specify CLI context:

$ docker context use rootless

root vs rootless:

  • root: DOCKER_HOST defaults to unix:///var/run/docker.sock (allows only local connections by the root user).
  • rootless: export DOCKER_HOST=unix://${XDG_RUNTIME_DIR}/docker.sock, e.g. unix:///run/user/123456/docker.sock

Storage

Storage Options

  • Storage Driver: for writing ephemeral data in container's writable layer, at a lower speed.
    • can also use tmpfs mount for ephemeral data to avoid writing into the container’s writable layer, which increases container's performance.
  • Volume: for write-intensive application, persistent data, shared data; better performance. The data is stored in Docker's storage directory (unlike bind mount below, which can be any absolute path on the host machine).
    • a volume doesn’t increase the size of the containers using it
  • Bind Mount: a file or directory on the host machine is mounted into a container. Similar to hostpath in k8s.

Overlay: lower vs upper vs merged

  • lower: Image layers, read-only.
  • upper: Container layers, read-write. Each running containers has a thin read-write layer. Do not persiste after the container is deleted.
  • merged: the union mount of the lower and upper. If a name exists in both, the object in 'upper' is visible while the object in 'lower' is hidden or merged.

Storage Driver

Storage Driver:

  • controls how images and containers are stored and managed on your Docker host.
  • allows you to write to the container’s writable layer.

Check which storage driver is being used:

$ docker info
...
Server:
 Containers: 24
 Images: 507
 Storage Driver: overlay2
  Backing Filesystem: extfs
...

Storage Driver - Overlay2

overlay2 is the preferred storage driver.

OverlayFS is a feature provided by Linux Kernel.

OverlayFS layers multiple directories on a single Linux host and presents them as a single directory.

Layers can be found in /var/lib/docker/overlay2

  • Each image and container layer has its own folder.
  • the special l directory contains shortened layer identifiers as symbolic links, to avoid hitting the page size limitation on arguments to the mount command. ls -l /var/lib/docker/overlay2/l

Check mounts: mount | grep overlay

Backing filesystem: ext4 or xfs.

Build

docker can build a docker image or an OCI image:

docker buildx build --output type=oci .
docker buildx build --output type=docker .

docker build vs docker buildx build

2 Docker build commands:

  • docker build: the legacy builder; always takes a copy of the local filesystem.
  • docker buildx build: Extended build capabilities with BuildKit.
    • BuildKit has been integrated to docker build since Docker 18.09.
    • BuildKit only requests the resources that the build needs, when they're needed.
    • A drop-in replacement for the legacy build.
    • In newer versions of Docker Desktop and Docker Engine, you're using Buildx by default when you invoke the docker build command.

BuildKit, or buildkitd, is the daemon process that executes the build workloads. A build execution starts with the invocation of a docker build command. Buildx interprets your build command and sends a build request to the BuildKit backend.

# List builder (will show BUILDKIT version)
$ docker buildx ls

# Inspect builder
$ docker buildx inspect $BUILD_NAME
$ docker buildx inspect default
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