Multi-stage Docker builds for minimal production images

Docker images are 1-2GB because they include build tools, dev dependencies, and source files.

Multi-stage: build in full image, copy output to minimal base. Node: FROM node:20 AS build, npm ci, npm run build. Then FROM node:20-slim, COPY --from=build. For Go: copy binary to scratch or distroless. Tips: use -slim or -alpine for final stage, use .dockerignore.

Build tools are only needed during build. Production container only needs runtime and compiled output. Multi-stage separates these in a single Dockerfile.