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Multi-stage build

Learn to produce small images with multi-stage build.

Multi-stage build

In a multi-stage build each FROM starts a new stage. The build stage can be large; the final stage takes only what is needed:

FROM golang:1.23 AS builder
WORKDIR /src
COPY . .
RUN CGO_ENABLED=0 go build -o /out/app .

FROM alpine:3.20
COPY --from=builder /out/app /usr/local/bin/app
CMD ["app"]

The image shrinks from ~800 MB to ~15 MB.

Use the cache wisely

Downloading dependencies in a separate layer speeds up repeated builds:

COPY go.mod go.sum ./
RUN go mod download
COPY . .
RUN go build -o /out/app .

With BuildKit, RUN --mount=type=cache,target=/root/.cache/go-build also keeps the build cache.

Practice Version information

Declare var version = "dev", to be set at build time with -ldflags "-X main.version=1.2.0", and print fmt.Println("sürüm:", version). (Without the flag it shows dev.)

Starter code

main.go
package main

import "fmt"

func main() {
    // buraya yaz
}

Sample solution

main.go
package main

import "fmt"

var version = "dev"

func main() {
    fmt.Println("sürüm:", version)
}

Expected output

sürüm: dev

Questions and answers

Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.

1 Why does <code>CGO_ENABLED=0</code> matter here?

  • It disables the GC
  • The binary doesn't link to libc and runs on any base Correct answer
  • Compilation gets faster
  • It disables tests

Explanation: With cgo on, the binary can link to the system C library; on alpine or scratch bases that library may differ or be missing.

2 Name the stage

Question 
FROM golang:1.23 ____ builder
Answer 
FROM golang:1.23 AS builder

Explanation: AS names the stage; later COPY --from=builder refers to it.

3 Stages

Term Meaning
builder Builds
final Only the binary
COPY --from Copy between stages
go mod download Cached layer

All lessons in Advanced

1 Reflection

  1. The reflect package
  2. reflect.Type
  3. reflect.Value
  4. Reflecting struct fields
  5. Dynamic call
  6. Summary - Reflection

2 unsafe

  1. The unsafe package
  2. unsafe.Pointer
  3. Sizeof and Alignof
  4. Offsetof
  5. unsafe and slice
  6. Summary - unsafe

3 cgo

  1. What is cgo?
  2. Calling C code
  3. C data types
  4. Go-C conversion
  5. cgo and memory
  6. Summary - cgo

4 plugin

  1. The plugin package
  2. Building a plugin
  3. Loading a plugin
  4. Plugin symbol
  5. Plugin and error
  6. Summary - plugin

5 database/sql

  1. The database/sql package
  2. sql.Open
  3. Query and Exec
  4. Prepared statement
  5. Transaction
  6. Summary - database/sql

6 Middleware

  1. The middleware concept
  2. HTTP middleware
  3. Middleware chain
  4. Logging middleware
  5. Auth middleware
  6. Summary - Middleware

7 gRPC

  1. What is gRPC?
  2. Protobuf definition
  3. Compiling with protoc
  4. gRPC server
  5. gRPC client
  6. Summary - gRPC

8 Project: Microservice

  1. Service skeleton
  2. Database layer
  3. HTTP API
  4. Docker and deployment

9 Docker

  1. Dockerfile
  2. Multi-stage build
  3. scratch image
  4. docker-compose
  5. Build args and labels
  6. Summary - Docker

10 pprof

  1. What is pprof?
  2. CPU profiling
  3. Memory profiling
  4. go test -cpuprofile
  5. The trace tool
  6. Summary - pprof

11 Observability

  1. log/slog
  2. Metrics
  3. Tracing
  4. OpenTelemetry
  5. Health check
  6. Summary - Observability

12 Advanced final

  1. Challenge 1 - Reflection
  2. Challenge 2 - Microservice
  3. Challenge 3 - Performance
  4. Challenge 4 - Database