Docker

What it is

Docker is a platform that allows packaging applications into containers — lightweight units that include the code, runtime, libraries, and configuration needed to run. A container runs identically on your laptop, in CI/CD, and in production.

Key concepts

Concept	Function	Example
Image	Immutable template with the filesystem	`node:22-alpine`, custom image
Container	Running instance of an image	`docker run my-app`
Dockerfile	Recipe for building an image	`FROM`, `COPY`, `RUN`, `CMD`
Registry	Image repository	Docker Hub, ECR, GHCR
Volume	Persistent storage outside the container	Database data, uploads

Basic Dockerfile

FROM node:20-alpine
WORKDIR /app
COPY package*.json ./
RUN npm ci --only=production
COPY . .
EXPOSE 3000
CMD ["node", "server.js"]

Docker vs VMs

Aspect	Docker	VM
Overhead	Minimal (shares kernel)	High (full OS)
Startup	Seconds	Minutes
Size	MBs	GBs
Isolation	Process	Hardware

Best practices

Use official and minimal base images (alpine)
Multi-stage builds for small images
Don't run as root
One process per container
.dockerignore to exclude unnecessary files

Why it matters

Docker standardized application packaging. A container works the same in development, CI, and production. This consistency eliminated an entire category of environment bugs and enabled practices like CI/CD, microservices, and immutable infrastructure.

References

Docker Documentation — Official documentation.
Dockerfile Best Practices — Official guide.
Docker Overview — Docker, 2024. Official Docker documentation.