Scenario

A Dockerfile at /home/interview/Dockerfile successfully builds a Go application, but the resulting Docker image is over 800MB in size due to including the full Go toolchain, build dependencies, and source code. Production images must be under 200MB.

Task

Rewrite the Dockerfile using a multi-stage build pattern to separate the build environment from the runtime environment. Use Alpine as the base image for the final runtime stage, ensure only the compiled binary and necessary runtime dependencies are copied to the final stage, build the optimized image with the tag myapp:fixed, and verify the final image size is below 200MB while maintaining full functionality.

Example

# Before (bloated image)

REPOSITORY    TAG       SIZE
myapp         original  550MB

# After (optimized multi-stage build)

REPOSITORY    TAG       SIZE
myapp         fixed     45.3MB

Step 1: Check the current image size

docker images

Shows the bloated image size myapp:original (over 500MB).

Step 2: Examine the existing Dockerfile

cat /home/interview/Dockerfile

The single-stage Dockerfile uses golang:1.21 as the base, which includes the entire Go toolchain (~800MB). The compiled binary is only a few MB, but everything else stays in the final image.

Step 3: Rewrite the Dockerfile with a multi-stage build

nano /home/interview/Dockerfile

FROM golang:1.21 AS builder

WORKDIR /app

COPY go.mod go.sum ./
RUN go mod download

COPY . .

RUN CGO_ENABLED=0 go build -o myapp .

FROM alpine:latest

WORKDIR /app
COPY --from=builder /app/myapp .

EXPOSE 8080

CMD ["./myapp"]

Step 4: Build the optimized image

docker build -t myapp:fixed /home/interview/

Step 5: Verify the size reduction

docker images

The fixed image should be under 50MB compared to the original 500MB+.