Docker

Docker is the tool that made containers mainstream. Before Docker (2013), Linux containers existed but required deep kernel knowledge to set up. Docker wrapped them in a simple CLI and an image format that anyone could use.

The core Docker workflow is three steps: build an image, push it to a registry, run it anywhere.

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A Dockerfile is a plain text recipe that describes how to build an image. Docker reads it top to bottom, executing each instruction and saving the result as a new layer.

The order of instructions matters for caching. Copying package.json before the rest of your source files means the npm ci layer gets cached and skipped on every rebuild where only your code changed. This keeps builds fast.

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An image is just a stack of layers plus some metadata (what command to run, what port to expose). Once built, you tag it and push it to a registry — a storage server for images.

Docker Hub is the public default. Companies typically run a private registry (AWS ECR, Google Artifact Registry, or a self-hosted Harbor) so images stay internal.

When a server pulls an image, it only downloads the layers it doesn't already have. If the base Node image is already cached locally, only your application layers transfer.

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docker run takes an image and starts a container from it. Key options engineers use daily:

• -d — run detached (in the background)
• -p 3000:3000 — map port 3000 on the host to port 3000 inside the container
• -e DATABASE_URL=... — inject environment variables
• --name my-api — give the container a name instead of the random default
• -v /data:/app/data — mount a host directory into the container (for persistent data)

A container runs until its main process exits. If the process crashes, the container stops. Docker has a --restart always flag that will restart it automatically, but in production you'd normally hand this responsibility to an orchestrator like Kubernetes.

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For local development with multiple services (an API, a database, a cache), Docker Compose lets you define them all in a single docker-compose.yml and start everything with one command: docker compose up. Each service gets its own container and they share a private network, so they can reach each other by name.

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• Docker getting started guide — the official tutorial, genuinely well-written
• Dockerfile best practices — how to write efficient, cache-friendly Dockerfiles
• Docker in 100 seconds (Fireship) — the fastest useful overview you will find
• Docker Compose overview — orchestrating multi-service local environments