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Docker Multi-stage Builds — Minimizing Image Size

This guide shows how to cut image size dramatically by stripping build dependencies with multi-stage builds, using Node.js, Go, and Java examples.

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Contents
  1. What Is a Multi-stage Build?
  2. Node.js Example
  3. Go Example (Static Binary)
  4. Java / Spring Boot Example
  5. Building a Specific Stage
  6. BuildKit Cache Optimization
  7. Image Size Comparison Tips

What Is a Multi-stage Build?

A technique that uses multiple FROM statements in a single Dockerfile so build tools stay in the first stage, and only the runtime artifacts are copied into the final image.

Regular build: build tools + source code + artifacts → large image Multi-stage: copy artifacts only → minimized image size

Node.js Example

Dockerfile
# ── Stage 1: Install dependencies + build ──────────────────
FROM node:20-alpine AS builder
WORKDIR /app

COPY package*.json ./
RUN npm ci --only=production

COPY . .
RUN npm run build

# ── Stage 2: Runtime image ──────────────────────────
FROM node:20-alpine AS runner
WORKDIR /app

ENV NODE_ENV=production

# Copy build artifacts only
COPY --from=builder /app/dist ./dist
COPY --from=builder /app/node_modules ./node_modules
COPY --from=builder /app/package.json ./

# Non-root user
RUN addgroup -S app && adduser -S app -G app
USER app

EXPOSE 3000
CMD ["node", "dist/server.js"]

Go Example (Static Binary)

Dockerfile
# ── Stage 1: Build ────────────────────────────────
FROM golang:1.22-alpine AS builder
WORKDIR /app

COPY go.mod go.sum ./
RUN go mod download

COPY . .
RUN CGO_ENABLED=0 GOOS=linux go build -o server ./cmd/server

# ── Stage 2: scratch (minimal image) ───────────────
FROM scratch
COPY --from=builder /app/server /server
COPY --from=builder /etc/ssl/certs/ca-certificates.crt /etc/ssl/certs/

EXPOSE 8080
ENTRYPOINT ["/server"]

scratch image: binary only, no OS — typically a few MB

Java / Spring Boot Example

Dockerfile
FROM eclipse-temurin:21-jdk AS builder
WORKDIR /app
COPY . .
RUN ./gradlew bootJar --no-daemon

FROM eclipse-temurin:21-jre AS runner
WORKDIR /app
COPY --from=builder /app/build/libs/*.jar app.jar
EXPOSE 8080
ENTRYPOINT ["java", "-jar", "app.jar"]

Building a Specific Stage

Bash
# Build only up to the builder stage (for debugging)
docker build --target builder -t myapp:builder .

# Final image
docker build -t myapp:latest .

BuildKit Cache Optimization

Dockerfile
# syntax=docker/dockerfile:1
FROM node:20-alpine AS builder
WORKDIR /app

# Copy package files first → reuse cache when source changes
COPY package*.json ./
RUN --mount=type=cache,target=/root/.npm \
    npm ci

COPY . .
RUN npm run build
Bash
# Build with BuildKit enabled
DOCKER_BUILDKIT=1 docker build -t myapp .

Image Size Comparison Tips

Bash
# Check image size
docker images myapp

# Analyze size by layer
docker history myapp:latest

# Analyze layers with the dive tool
docker run --rm -it \
  -v /var/run/docker.sock:/var/run/docker.sock \
  wagoodman/dive myapp:latest
#docker#dockerfile
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