Reducing Docker Image Size: Step-by-Step Checklist

Bloated images slow CI/CD and waste bandwidth. Optimize with intent.

1. Start with a Baseline

docker build -t app:baseline .
docker image inspect app:baseline --format='{{.Size}}'
Copy code

Record MB size.

2. Choose a Smaller Base

Prefer:

• node:20-alpine over node:20
• python:3.12-slim over python:3.12

3. Multi-Stage Builds

FROM node:20-alpine AS build
WORKDIR /app
COPY package*.json ./
RUN npm ci --omit=dev
COPY . .
RUN npm run build

FROM node:20-alpine AS runtime
WORKDIR /app
COPY --from=build /app/.next ./.next
COPY --from=build /app/node_modules ./node_modules
COPY package.json .
CMD ["node", "server.js"]
Copy code

4. Prune Dev Dependencies

Use npm ci --omit=dev (or pnpm install --prod).

5. Collapse Layers

Combine adjacent RUNs:

RUN apk add --no-cache libc6-compat && \
    adduser -D app && \
    chown -R app:app /app
Copy code

6. Remove Build Artifacts

• Delete unused docs/tests.
• Strip source maps in final image if not needed.

7. Use .dockerignore

Add:

.git
node_modules
Dockerfile*
*.log
coverage
Copy code

8. Scan for Bloat

dive app:baseline
Copy code

Look for large layers or vendored blobs.

9. Add Health & User Hardening

USER app
HEALTHCHECK CMD wget -qO- http://localhost:3000/health || exit 1
Copy code

10. Measure Again

Rebuild and compare size delta. Target >30% reduction.

Ship leaner, ship faster.