05 — Create a .dockerignore file

Category: Dockerfile · Priority: Medium · Step: 6

Exclude: node_modules/, .env, .git, __pycache__, *.log. Keeps the build context lean.

🎯 What is this task?

When Docker builds your image, it copies everything in your project folder into the build context. This includes things you don't need inside the container — like node_modules, .env files, the .git folder, and log files. A .dockerignore file tells Docker to skip those. It works exactly like a .gitignore file.

Without this, Docker might copy hundreds of MB of files unnecessarily, and could even leak secret .env values into your image.

🤖 AI Prompt

"Create a .dockerignore file for a Node.js project. Exclude: node_modules, .env files, .git folder, log files, editor config files like .vscode, test files, and any OS junk like .DS_Store. Add a comment above each group explaining why it's excluded."

📋 Step-by-step

Step 1 — Create .dockerignore in your project root

This file goes in the same folder as your Dockerfile. Create a file named exactly .dockerignore:

# Dependencies — Docker will install these fresh via npm install
node_modules
npm-debug.log

# Environment secrets — NEVER bake these into an image
.env
.env.*
*.env

# Git history — not needed inside container
.git
.gitignore

# Editor files
.vscode
.idea
*.swp

# OS junk
.DS_Store
Thumbs.db

# Logs
logs/
*.log

# Test files
__tests__/
*.test.js
*.spec.js

Step 2 — Verify it works by checking build output

docker build -t my-docker-app:v4 .

Look at the first line of output — it shows [+] Building X.Xs (Y steps). The "Sending build context" step should be small (a few KB, not MB).

Step 3 — Confirm node_modules wasn't copied

docker run --rm my-docker-app:v4 ls /app

You'll see index.js, package.json, and node_modules — but the node_modules inside was installed by npm install in the Dockerfile, not copied from your machine.

✅ Done when

• .dockerignore file exists in your project root
• Build context size in docker output is small
• .env files and node_modules from your machine are not baked into the image