What is Docker and why is it used?

Docker is a platform for building, running, and shipping applications in containers — lightweight, isolated environments that package the application code together with all its dependencies (runtime, libraries, config).

The core problem Docker solves

"It works on my machine" — Docker eliminates environment inconsistencies by making the environment part of the artifact. The same container image runs identically on a developer's laptop, a CI server, and production.

Key concepts

• Image — a read-only template (snapshot) of a file system with your app and dependencies. Built from a Dockerfile.
• Container — a running instance of an image. Isolated from the host and other containers via Linux namespaces and cgroups.
• Dockerfile — a text file with instructions for building an image:

FROM node:20-alpine
WORKDIR /app
COPY package*.json ./
RUN npm ci
COPY . .
RUN npm run build
CMD ["node", "dist/index.js"]

• Registry — a repository for storing and sharing images. Docker Hub is the public default; AWS ECR, Google Artifact Registry, and GitHub Container Registry are common private options.
• Docker Compose — a tool to define and run multi-container applications (e.g., app + database + cache) with a single docker-compose.yml.

Containers vs. VMs

Virtual machines virtualize hardware and run a full OS per VM — heavy (GBs). Containers share the host OS kernel — lightweight (MBs), start in seconds, and run thousands per host.

Common use cases:

• Consistent dev environments across the team
• CI/CD pipelines that build and test in isolation
• Microservices deployment
• Running services locally without installing them (postgres, redis, etc.)