How to Reduce Docker Image Size With a Multi-Stage Build in CI

A multi-stage build keeps compilers and dev dependencies out of the final image by copying only the built output forward.

Compile in a heavier builder stage, then COPY --from=builder only the runtime artifacts into a minimal base. The toolchain layers are discarded, shrinking the published image and its attack surface.

Steps

• Name the build stage with FROM ... AS builder.
• Install dependencies and build in that stage.
• Start a slim final stage and COPY --from=builder only what runtime needs.

Dockerfile

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

FROM node:20-bookworm-slim
WORKDIR /app
COPY --from=builder /app/node_modules ./node_modules
COPY --from=builder /app/dist ./dist
CMD ["node", "dist/server.js"]

Gotchas

• Only the final stage is published; earlier stages exist only to feed COPY --from.
• A -slim or distroless base trims the runtime further, but verify required shared libraries are present.

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