Complete Beginner's Guide to Docker

Docker packages an application together with everything it needs to run (code, runtime, system tools, libraries) into a single unit called a container. This guide covers the core concepts and gets you running your first container.

Why Docker exists

Before containers, “it works on my machine” was a real and constant problem: an app might depend on a specific version of a language runtime, a system library, or an environment variable that only existed on one developer’s laptop. Docker solves this by packaging the entire runtime environment, so the same container behaves the same way everywhere.

Key concepts

• Image: a read-only template that defines what goes into a container (like a class in programming).
• Container: a running instance of an image (like an object created from that class).
• Dockerfile: a text file with instructions for building an image.
• Registry: a place to store and share images, like Docker Hub.

Installing Docker

Download Docker Desktop for macOS or Windows, or install Docker Engine directly on Linux. Verify the install with:

docker --version

Running your first container

Pull and run a simple web server:

docker run -d -p 8080:80 nginx

This downloads the official nginx image if you don’t already have it, starts a container in the background (-d), and maps port 8080 on your machine to port 80 inside the container. Visit http://localhost:8080 and you should see the default nginx welcome page.

Writing a Dockerfile

For your own application, you’ll typically write a Dockerfile rather than relying on a public image alone:

FROM node:20-alpine
WORKDIR /app
COPY package*.json ./
RUN npm install
COPY . .
EXPOSE 3000
CMD ["npm", "start"]

Build and run it with:

docker build -t my-app .
docker run -p 3000:3000 my-app

Common commands you’ll actually use

• docker ps: list running containers.
• docker ps -a: list all containers, including stopped ones.
• docker stop <container>: stop a running container.
• docker logs <container>: view a container’s output, useful for debugging.
• docker system prune: clean up unused images and containers to free disk space.

Where to go from here

Once running a single container feels comfortable, the natural next step is Docker Compose, which lets you define and run multi-container setups (an app, a database, a cache) from a single YAML file.