Summary - Docker
- Go course
- Advanced
- Lesson 52/68
- 4 min
Combine multi-stage and scratch to write a minimal image.
Docker summary
The recipe for a good Go image: a multi-stage build, copying go.mod first to keep the cache, a static binary with CGO_ENABLED=0, a scratch or distroless base, version info with ARG + ldflags, metadata with LABEL, and Compose for the local development setup.
docker build --build-arg VERSION=1.0.0 -t app:1.0.0 .
docker run -p 8080:8080 app:1.0.0The shutdown signal
docker stop sends SIGTERM to the process and kills it with SIGKILL after 10 seconds. The Go service should catch this signal and finish in-flight requests:
ctx, stop := signal.NotifyContext(context.Background(), syscall.SIGTERM, os.Interrupt)
defer stop()
go srv.ListenAndServe()
<-ctx.Done()
srv.Shutdown(context.Background())Also use the exec form (ENTRYPOINT ["/app"]); with the shell form the signal may not reach the program.
Practice Shutdown on signal
Create ctx, cancel := context.WithCancel(context.Background()) and call cancel() in a goroutine to imitate the signal. Wait with <-ctx.Done(), then print fmt.Println("kapanıyor").
Starter code
package main
import (
"context"
"fmt"
)
func main() {
// buraya yaz
}Sample solution
package main
import (
"context"
"fmt"
)
func main() {
ctx, cancel := context.WithCancel(context.Background())
go cancel()
<-ctx.Done()
fmt.Println("kapanıyor")
}Expected output
kapanıyor
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Which instruction runs the container as a non-root user?
- EXPOSE
- USER Correct answer
- RUN
- ARG
Explanation: USER runs following commands and the container process as the given user; distroless provides a ready "nonroot" user.
2 Non-root user
FROM gcr.io/distroless/static
COPY --from=builder /out/app /app
____ nonroot
ENTRYPOINT ["/app"]FROM gcr.io/distroless/static
COPY --from=builder /out/app /app
USER nonroot
ENTRYPOINT ["/app"]Explanation: USER nonroot keeps the container from running with root privileges.
3 Group summary
| Term | Meaning |
|---|---|
Multi-stage |
Small image |
CGO_ENABLED=0 |
Static binary |
SIGTERM |
Graceful shutdown |
USER nonroot |
Safer |