What is Docker Multi-Stage Build?

Question

Accepted Answer

**Multi-Stage Build** — multiple build stages in a single Dockerfile for smaller final images.

```dockerfile
# Stage 1: Build
FROM node:20-alpine AS builder
WORKDIR /app
COPY package*.json ./
RUN npm ci --only=production
COPY . .
RUN npm run build

# Stage 2: Test
FROM node:20-alpine AS test
WORKDIR /app
COPY --from=builder /app .
RUN npm ci
RUN npm run test

# Stage 3: Production
FROM node:20-alpine AS production
RUN addgroup -S appgroup && adduser -S appuser -G appgroup
WORKDIR /app
COPY --from=builder /app/dist ./dist
COPY --from=builder /app/node_modules ./node_modules
COPY package*.json ./
USER appuser
EXPOSE 3000
HEALTHCHECK --interval=30s --timeout=3s \
  CMD wget -qO- http://localhost:3000/health || exit 1
CMD ["node", "dist/main.js"]

# Stage 4: Debug (only used with --target debug)
FROM production AS debug
USER root
RUN apk add --no-cache curl bash
CMD ["sh", "-c", "node dist/main.js & sleep infinity"]
```

```bash
# Build specific stage
docker build --target production -t myapp:latest .
docker build --target debug -t myapp:debug .

# Copy from specific stage
COPY --from=builder /app/dist ./dist
COPY --from=0 /app/dist ./dist  # by index
```

**Benefits:**
- Smaller final image.
- Better caching.
- Security (no build tools in prod).
- Single Dockerfile for all environments.

DevOps: What is Docker Multi-Stage Build?