What is Docker?

Package your app into a container — has everything it needs, runs anywhere.

• Dockerfile = Recipe
• Image = Packed box (built from recipe)
• Container = Box running (live app)

Simple Dockerfile (Practice)

FROM node:18-alpine       # Base image
WORKDIR /app              # Working folder inside container
COPY . .                  # Copy your files in
RUN npm install           # Install dependencies
EXPOSE 3000               # Declare port
CMD ["node", "src/index.js"]  # Start the app

Production Dockerfile (2-Stage Build)

# Stage 1: BUILD
FROM node:18-alpine AS build
WORKDIR /app
COPY package*.json ./
RUN npm install
COPY . .
RUN npm run build         # Compiles React → HTML/CSS/JS

# Stage 2: SERVE
FROM nginx:alpine
COPY --from=build /app/build /usr/share/nginx/html
EXPOSE 80
CMD ["nginx", "-g", "daemon off;"]

Why 2 stages? Build stage compiles code, then gets thrown away. Only the final HTML/CSS/JS is served — smaller, cleaner, safer.

daemon off; keeps Nginx in foreground so Docker knows the container is still alive.

Dockerfile Order (Matters!)

FROM        # 1. Always first
WORKDIR     # 2. Set folder
COPY        # 3. Copy files
RUN         # 4. Install / build
EXPOSE      # 5. Declare port
CMD         # 6. Always last

Caching Rule — Put stable things first, changing things last

# ✅ Good (cache friendly)
COPY package*.json ./   # rarely changes → cached
RUN npm install
COPY . .                # your code changes often → at the bottom

# ❌ Bad (slow — reruns npm install on every code change)
COPY . .
RUN npm install

What is Nginx?