Docker Cheatsheet¶

Useful Links¶

Concepts¶

A Docker image is a read-only template. For example, an image could contain an Ubuntu operating system with Apache and your web application installed. Images are used to create Docker containers. Docker provides a simple way to build new images or update existing images, or you can download Docker images that other people have already created. Docker images are the buildcomponent of Docker.

Docker registries hold images. For orchestration, see Kubernetes Concepts.

Cheatsheet¶

To show only running containers, use the following command.

docker ps

To show all containers, use the following command.

docker ps -a

To show the last started container, use the following command.

docker ps -l

To download an image from Docker Hub, use the docker pull command.

docker pull centos

Create then start a container: docker run [OPTIONS] IMAGE [COMMAND] [ARG...] * Docker run reference

docker run hello-world

Run with interactive terminal (i = interactive t = terminal):

docker run -it ubuntu bash

Start then detach the container (daemonize):

docker run -d -p8088:80 --name webserver nginx

If you want a transient container, docker run --rm will remove the container after it stops.

To look inside the container, use the logs command. You can also use -f to act like tail -f.

docker logs <container name>

Stop container:

docker stop <container name>   # Container ID or name.

Delete container:

docker rm <container name>

To check the environment:

docker run -it alpine env

Docker version / info:

docker version
docker info

Port Mapping¶

-p 80:5000 would map port 80 on our local host to port 5000 inside our container.

docker run -d -p 80:5000 training/webapp python app.py

Full format: ip:hostPort:containerPort | ip::containerPort | hostPort:containerPort | containerPort

docker run -d -p 127.0.0.1:80:5000 training/webapp python app.py

Both hostPort and containerPort can be specified as a range of ports. When specifying ranges for both, the number of container ports in the range must match the number of host ports in the range, for example: -p1234-1236:1234-1236/tcp

The -P flag tells Docker to map any required network ports inside our container to our host (using random ports).

docker run -d -P training/webapp python app.py

Linking¶

--link <name or id>:alias where name is the name of the container we’re linking to and alias is an alias for the link name. The --link flag also takes the form: --link <name or id>

docker run -d --name myES -p 9200:9200 -p 9300:9300 elasticsearch
docker run --name myK --link myES:elasticsearch  -p 5601:5601 -d docker-kibana-sense

Networks¶

docker network ls

Find out the container’s IP address:

docker network inspect bridge

Data Volumes¶

Create a new volume inside a container at /webapp:

docker run -d -P --name web -v /webapp training/webapp python app.py
docker inspect web

You can also use the VOLUME instruction in a Dockerfile to add one or more new volumes to any container created from that image.

Mount the host directory, /src/webapp, into the container at /opt/webapp.

docker run -d -P --name web -v /src/webapp:/opt/webapp training/webapp python app.py

On Windows, use: docker run -v /c/Users/<path>:/<container path> ...

Example Dockerfile¶

vim Dockerfile

FROM docker/whalesay:latest
RUN apt-get -y update && apt-get install -y fortunes
CMD /usr/games/fortune -a | cowsay

docker build -t docker-whale .

Multi-stage Build Example¶

The following is a Dockerfile example showing a multi-stage build. Multi-stage builds are useful to optimize image sizes, separating the build environment from the runtime environment.

# Build stage.
FROM golang:1.22-alpine AS builder
WORKDIR /app
COPY go.mod go.sum ./
RUN go mod download
COPY . .
RUN go build -o myapp .

# Final stage.
FROM alpine:latest
WORKDIR /root/
COPY --from=builder /app/myapp .
CMD ["./myapp"]

Docker Compose Example¶

compose.yaml (or compose.yaml) to define and run multi-container applications:

version: "3.8"

services:
  web:
    image: nginx:alpine
    ports:
      - "8080:80"
    volumes:
      - ./nginx.conf:/etc/nginx/nginx.conf
    depends_on:
      - api

  api:
    build: ./api
    environment:
      - DB_HOST=db
      - DB_USER=postgres
    ports:
      - "5000:5000"

  db:
    image: postgres:16-alpine
    environment:
      - POSTGRES_PASSWORD=secret
    volumes:
      - pgdata:/var/lib/postgresql/data

volumes:
  pgdata:

Run compose:

docker compose up -d
docker compose down

Docker Cleanup Commands¶

To remove all unused or dangling images, containers, volumes, and networks:

docker system prune -a --volumes

Multi-architecture Builds¶

Use buildx to build for multiple architectures (e.g., AMD64 and ARM64):

docker buildx create --use
docker buildx build --platform linux/amd64,linux/arm64 -t myrepo/myimage:latest --push .