What is Docker and how does it work?

Short answer

Docker is an open-source platform that automates the deployment of applications inside lightweight, portable containers. Containers package an application with all its dependencies, ensuring consistency across development, staging, and production environments.

How it works

Docker uses the host operating system kernel through Linux features like cgroups and namespaces to isolate processes. Unlike virtual machines, containers share the host OS kernel, making them significantly lighter and faster to start. The Docker Engine manages container lifecycle, networking, and storage through a client-server architecture.

Example

A simple Python application can be containerized with the following Dockerfile:

FROM python:3.11-slim
WORKDIR /app
COPY requirements.txt .
RUN pip install -r requirements.txt
COPY . .
CMD ["python", "app.py"]

Build and run it:

docker build -t myapp .
docker run -p 5000:5000 myapp

Why it matters

Docker eliminates the "it works on my machine" problem by providing a consistent runtime environment. It accelerates CI/CD pipelines, simplifies microservices architecture, and improves resource utilization compared to traditional VMs. This has made Docker a foundational technology in modern DevOps practices.