Docker for Beginners: Images, Containers and Volumes

Docker packages an application with everything it needs (runtime, libraries, config) into an image. Running an image creates a container: an isolated process that behaves the same on your laptop, a CI runner and a production server.

Images vs containers

• An image is a read-only template, built from a Dockerfile and made of layers.
• A container is a running (or stopped) instance of an image, with its own writable layer.
• One image can run as many containers.

Commands you'll use every day

docker run -d --name web -p 8080:80 nginx       # run in background, host 8080 -> container 80
docker ps                                        # running containers
docker logs -f web                               # follow logs
docker exec -it web sh                           # shell inside the container
docker stop web && docker rm web

docker images                                    # local images
docker pull postgres:17
docker system prune                              # clean up unused data

Environment variables

docker run -d --name db \
  -e POSTGRES_PASSWORD=secret \
  -e POSTGRES_DB=app \
  -p 5432:5432 \
  postgres:17

Volumes: keep data when containers are removed

A container's filesystem disappears with the container. Put data that must survive in a volume:

docker volume create pgdata
docker run -d --name db -e POSTGRES_PASSWORD=secret \
  -v pgdata:/var/lib/postgresql/data postgres:17

# Bind mount: share a host folder (great for development)
docker run --rm -v "$PWD":/app -w /app node:22 npm test

Your first image

FROM node:22-alpine
WORKDIR /app
COPY package*.json ./
RUN npm ci --omit=dev
COPY . .
CMD ["node", "server.js"]

docker build -t my-api:1.0 .
docker run -d -p 3000:3000 my-api:1.0

Containers are not virtual machines: they share the host's kernel, which is why they start in milliseconds and use little memory.