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Free Docker cheat sheet

Docker cheat sheet

Docker on one page: images, containers, the Dockerfile, docker compose, and the cleanup commands you forget.

ImagesRun containersManage containersA basic Dockerfiledocker compose (multi-container)Clean up

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Docker Cheat Sheet

Quick reference from FreeCodingCourses.com

Images

docker pull node:20: download an image from a registry.
docker build -t myapp .: build an image from the Dockerfile here, tag it myapp.
docker images: list local images.
docker rmi myapp: remove an image.

Run containers

docker run myapp: start a container from an image.
docker run -p 3000:3000 myapp: map host port 3000 to container port 3000.
docker run -d myapp: detached: run in the background.
docker run -it ubuntu bash: interactive shell inside the container.
docker run --env-file .env myapp: pass environment variables.

Manage containers

docker ps: running containers. docker ps -a includes stopped.
docker logs -f <id>: view and follow a container's output.
docker exec -it <id> bash: open a shell in a running container.
docker stop <id> docker start <id>: stop and restart.
docker rm <id>: delete a stopped container.

A basic Dockerfile

FROM node:20: start from a base image.
WORKDIR /app: set the working directory inside the image.
COPY package*.json ./ RUN npm ci: copy deps first, install, so this layer caches.
COPY . .: copy the rest of your code.
CMD ["node", "server.js"]: the command that runs when the container starts.

docker compose (multi-container)

docker compose up -d: start everything in compose.yaml, detached.
docker compose down: stop and remove the containers.
docker compose logs -f: tail logs from all services.
docker compose ps: status of the compose services.

Clean up

docker system prune: remove stopped containers, unused networks, dangling images.
docker image prune -a: remove all unused images. Frees a lot of disk.
docker volume ls: list volumes (your persistent data lives here).

Gotchas worth knowing

•Copy package.json and install dependencies before copying the rest of your code. Docker caches layers, so your slow install step is reused whenever only your source changed.
•Containers are disposable and lose their internal changes when removed. Anything that must survive, like a database, belongs in a named volume.
•docker system prune reclaims disk from stopped containers and dangling images. Run it when Docker starts eating your drive, but know it deletes stopped containers.
•-p host:container maps ports. If you cannot reach your app in the browser, you probably forgot the -p flag or mapped the wrong port.

Docker questions, answered

What is the difference between an image and a container?

An image is a read-only template: your app plus everything it needs to run, frozen. A container is a running instance of an image. One image can start many containers, the same way one class can create many objects.

Why use Docker at all?

Docker packages your app with its exact environment, so it runs the same on your laptop, a teammate's machine, and the server. It ends the it works on my machine problem and makes deploying and running multiple services much simpler.

Is it docker-compose or docker compose?

Both exist. The older standalone tool was docker-compose with a hyphen. Modern Docker builds it in as a subcommand, docker compose with a space. The commands and the YAML file are otherwise the same; prefer the space version on new setups.