Docker

This lesson covers Docker. You do not need to memorize everything. Understand the flow first.

What you will learn

• What docker means — in normal words, not textbook words
• How it works step by step
• Code you can run today on your laptop
• Where teams use this in real projects

Before you start

• Software: Node.js LTS from nodejs.org, VS Code, and a terminal
• Knowledge: Earlier lessons in this Node.js course
• Previous lesson: API Testing — Complete Guide

Explain it simply

Docker packages your app and Node runtime into a container that runs the same on your laptop and in the cloud.

Why developers use this

• No "works on my machine" problems
• Easy deploy to Railway, AWS, Azure
• Standard in modern teams

How it works (step by step)

1. Tests pass on your machine.
2. Build a Docker image or set Node version on the host.
3. Set environment variables on the server (never commit secrets).
4. Hit the health URL and watch logs for the first real users.

Code example — type this yourself

FROM node:22-alpine
WORKDIR /app
COPY package*.json ./
RUN npm ci --omit=dev
COPY . .
EXPOSE 3000
CMD ["node", "server.js"]

Build with docker build -t my-api . and run with docker run -p 3000:3000 my-api.

What each part does

• FROM node:22-alpine — Line 1: runs as written.
• WORKDIR /app — Line 2: runs as written.
• COPY package*.json ./ — Line 3: runs as written.
• RUN npm ci --omit=dev — Line 4: runs as written.
• COPY . . — Line 5: runs as written.
• EXPOSE 3000 — Line 6: runs as written.
• CMD ["node", "server.js"] — Line 7: runs as written.

Real life: where Docker shows up

After tests pass locally, the team uses Docker to ship the same build to staging, then production, with the same Node version everywhere. In interviews, explain the trade-off you chose and what you would measure in production.

Try it yourself — hands-on

1. Add Dockerfile to a small Express app
2. Create .dockerignore with node_modules
3. Build and run the container

Common mistakes (avoid these)

• COPY . . before npm ci — slow rebuilds; copy package.json first.

Interview note

Senior interviews may ask how Docker behaves under load, failure, or security review — mention logging, timeouts, and validation.

Summary

• Dockerfile describes how to build the image
• npm ci installs exact versions from lock file
• Map port 3000 with -p