"It works on my machine." Few phrases have caused as much friction between developers and operations teams. Docker was created precisely to end that problem. Today, it is the foundation of CI/CD pipelines, cloud environments and Kubernetes. In this guide, we explain what is docker, how it works and how to get started.
What is Docker
Docker is a platform for building, shipping and running applications in containers. A container packages the code together with everything it needs to run: libraries, dependencies, configuration and the exact language version.
As a result, the application behaves the same on a developer's laptop, on the test server and in production.
Container or virtual machine?
Both isolate applications, but in different ways:
- Virtual machine: emulates an entire computer with a full operating system. It is heavy, takes up gigabytes and needs minutes to boot.
- Container: shares the host's kernel and isolates only the process. It is lightweight, takes up megabytes and starts in seconds.
That is why a single server can run dozens of containers instead of a handful of virtual machines.
Core concepts
Image
An image is a read-only template containing the application and its dependencies. It works like a recipe.
Container
A container is a running instance of an image. From a single image, you can start several identical containers.
Dockerfile
It is the text file with the instructions to build the image. A simple example for a Node.js application:
FROM node:22-alpine WORKDIR /app COPY package*.json ./ RUN npm ci --omit=dev COPY . . CMD ["node", "server.js"]
Registry
It is the repository where images are stored, such as Docker Hub, GitHub Container Registry or Amazon ECR.
Docker Compose
It lets you describe several services in a single YAML file. With one command, you start the application, the database and Redis together. It is ideal for development environments.
Basic commands
docker build -t my-app .: builds an image from the Dockerfile.docker run -p 8080:80 my-app: starts a container and exposes the port.docker ps: lists running containers.docker logs -f name: follows a container's logs.docker compose up -d: starts every service defined in Compose.
Benefits of Docker
- Consistency: the same environment from development to production.
- Fast onboarding: a new developer runs the project with a single command.
- Predictable deployments: you ship a tested image, not a list of manual steps.
- Scalability: orchestrators such as Kubernetes and ECS start new replicas in seconds.
- Isolation: projects with different PHP, Node or Python versions coexist on the same server.
Best practices for production
- Use official, lean base images, such as the alpine or slim variants.
- Apply multi-stage builds to separate compilation from the final image.
- Do not run processes as root inside the container.
- Never bake passwords and keys into the image. Instead, use environment variables or a secrets manager.
- Pin base image versions and scan for vulnerabilities with tools such as Trivy.
- Store persistent data in volumes, since a container can be destroyed at any time.
Frequently asked questions
Are Docker and Kubernetes competitors?
No. Docker builds and runs containers. Kubernetes orchestrates many containers across several servers, handling scaling, networking and failure recovery.
Does Docker work on Windows?
Yes. Docker Desktop uses WSL 2 to run Linux containers on Windows with good performance.
Conclusion
Understanding what Docker is is the first step toward modernizing how a company builds and ships software. Containers bring consistency, speed and scalability. If you want to containerize your systems or set up an automated deployment pipeline, WDS Developer can help.
