Docker Image Optimization Guide

# Image size
docker images my-app

# Layer breakdown
docker history my-app:latest

# Detailed analysis with dive
docker run --rm -it -v /var/run/docker.sock:/var/run/docker.sock wagoodman/dive my-app:latest

# Stage 1: Build (1.1GB with all dev tools)
FROM node:22 AS builder
WORKDIR /app
COPY package*.json ./
RUN npm ci
COPY . .
RUN npm run build

# Stage 2: Production (180MB)
FROM node:22-alpine
WORKDIR /app
COPY package*.json ./
RUN npm ci --production
COPY --from=builder /app/dist ./dist
EXPOSE 3000
CMD ["node", "dist/server.js"]

FROM golang:1.23 AS builder
WORKDIR /app
COPY go.* ./
RUN go mod download
COPY . .
RUN CGO_ENABLED=0 go build -o server .

FROM scratch
COPY --from=builder /app/server /server
EXPOSE 8080
ENTRYPOINT ["/server"]

# ✅ Good — dependencies change less often than code
FROM node:22-alpine
WORKDIR /app

# Layer 1: Dependencies (cached until package.json changes)
COPY package*.json ./
RUN npm ci --production

# Layer 2: Application code (changes every commit)
COPY . .

CMD ["node", "server.js"]

# ❌ Bad — COPY . . before npm ci invalidates cache on every code change
FROM node:22-alpine
WORKDIR /app
COPY . .
RUN npm ci --production
CMD ["node", "server.js"]

node_modules
.git
.github
*.md
.env*
.vscode
coverage
dist
.next
docker-compose*.yml
Dockerfile*

# ❌ Bad — 3 layers, apt cache remains
RUN apt-get update
RUN apt-get install -y curl wget
RUN apt-get clean

# ✅ Good — 1 layer, cache cleaned
RUN apt-get update && \
    apt-get install -y --no-install-recommends curl wget && \
    rm -rf /var/lib/apt/lists/*

# syntax=docker/dockerfile:1
FROM node:22-alpine
WORKDIR /app
COPY package*.json ./

# Cache npm download between builds
RUN --mount=type=cache,target=/root/.npm \
    npm ci --production

COPY . .
CMD ["node", "server.js"]

RUN --mount=type=cache,target=/root/.cache/pip \
    pip install -r requirements.txt

# ❌ Breaks randomly when latest changes
FROM node:latest

# ✅ Reproducible builds
FROM node:22.12-alpine3.21

# GitHub Actions with BuildKit cache
- uses: docker/build-push-action@v6
  with:
    push: true
    tags: my-app:${{ github.sha }}
    cache-from: type=gha
    cache-to: type=gha,mode=max