Infrastructure & Operations › Containers

Docker

The most common tool for building and running containers.

Also known as: Docker, docker container, Docker Engine, Docker CLI, dockerize

Docker is the most widely used tool for building and running containers: packaged applications that carry their dependencies with them and run isolated from the rest of the system. “It works on my machine” becomes “it works wherever the container runs”.

Two concepts to keep apart:

• An image is the packaged, read-only template: your code, runtime and libraries (Docker image).
• A container is a running instance of an image (container).

A simple Dockerfile describes how to build an image:

FROM python:3.12-slim
WORKDIR /app
COPY requirements.txt .
RUN pip install -r requirements.txt
COPY . .
CMD ["python", "app.py"]

(See Dockerfile.)

Everyday commands

docker build -t myapp .                  # build an image from the Dockerfile in this folder
docker run -p 8080:8000 myapp            # run it, mapping host port 8080 to container port 8000
docker run -d --name web myapp           # run in the background
docker ps                                # list running containers  (-a: include stopped)
docker logs -f web                       # view output
docker exec -it web sh                   # open a shell inside it
docker stop web && docker rm web

Things to know early

• Containers are disposable. Their filesystem disappears when the container is removed. Keep data you need in a volume or an external database (ephemeral filesystem).
• Ports must be published (-p host:container) to be reachable from outside (port mapping). Inside the container, a server should listen on 0.0.0.0, not 127.0.0.1.
• Layers are cached. Order Dockerfile steps from least to most frequently changed (install dependencies before copying your source), so rebuilds are fast (image layers).
• Add a .dockerignore to keep .git, node_modules and local .env files out of the image.
• Don’t bake secrets into images. They stay in the layers. Pass them at run time.
• Use Docker Compose to run an app with its database and other services together.
• A container isn’t a virtual machine: it shares the host’s kernel (containers vs VMs).

Docker is one implementation of a standard (OCI). Other tools can run the same images.