Glossary

What Is Docker?

Docker Containerization Platform

Docker is a platform for building, running, and distributing containerized applications. A Docker container packages your code with all its dependencies into a standardized unit that runs identically everywhere — your laptop, your teammate's laptop, CI, staging, and production.

How Docker Works

You write a Dockerfile that describes your app's environment: base image, dependencies, code, and start command. Docker builds an image from this file. You run the image as a container. Share the image via Docker Hub or a private registry. It works everywhere.

Docker Compose lets you define multi-container applications in a YAML file. Run docker compose up and your app, database, cache, and any other services start together with proper networking. This is the standard for local development environments.

Docker revolutionized deployment by solving 'it works on my machine.' The same container image runs in development, CI, staging, and production. No more environment drift, dependency conflicts, or configuration surprises.

Why Developers Use Docker

Docker is a foundational DevOps tool. Every cloud platform runs containers. Every CI/CD pipeline builds container images. Learning Docker is essential for any developer who deploys applications.

Key Concepts

  • Dockerfile — A script that defines how to build a container image — FROM, RUN, COPY, EXPOSE, CMD instructions
  • Image — A read-only template containing your application, dependencies, and OS — built from a Dockerfile
  • Container — A running instance of an image — isolated, lightweight, and disposable
  • Docker Compose — Tool for defining multi-container applications — docker-compose.yml describes your entire stack
  • Volume — Persistent storage that survives container restarts — mount for databases, uploads, and logs
  • Layer Caching — Docker caches each Dockerfile instruction's result — only re-runs steps that changed

Dockerfile for a Node.js App

dockerfile
FROM node:20-alpine
WORKDIR /app

# Install dependencies first (layer caching)
COPY package*.json ./
RUN npm ci --production

# Copy source code
COPY . .

EXPOSE 3000
CMD ["node", "server.js"]

# docker-compose.yml
# services:
#   app:
#     build: .
#     ports: ["3000:3000"]
#   db:
#     image: postgres:16
#     environment:
#       POSTGRES_PASSWORD: secret

Learn Docker — Top Videos

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Frequently Asked Questions

Do I need Docker for development?

Strongly recommended. Docker ensures consistent environments across your team and matches production. Docker Compose lets you run databases, caches, and services locally without installing them on your machine.

Docker vs virtual machines?

Docker containers share the host OS kernel — they're lightweight (MBs), start fast (seconds), and use less resources. VMs include a full OS — they're heavier (GBs) and slower to start (minutes). Use containers for applications, VMs for OS-level isolation.

How do I learn Docker?

Start with Dockerizing a simple web app (write a Dockerfile, build, run). Then use Docker Compose for multi-service setups. Then learn image optimization (multi-stage builds, layer caching). Practice with your own projects.

Want a structured learning path?

Plan a Docker Lesson →

Related Terms

ContainerKubernetesDevOpsCI/CDYAML

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