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Build args and labels

Learn to customize the image with ARG and LABEL.

Build args and labels

ARG takes a value from outside at build time; LABEL adds metadata to the image:

ARG VERSION=dev
RUN go build -ldflags "-X main.version=${VERSION}" -o /out/app .

LABEL org.opencontainers.image.version=$VERSION \
      org.opencontainers.image.source="https://github.com/ornek/app"

Build: docker build --build-arg VERSION=1.4.0 -t app:1.4.0 .

Don't pass secrets with ARG

ARG values can appear in docker history output; they aren't suitable for passwords and tokens. BuildKit's secret mount is used instead:

RUN --mount=type=secret,id=netrc,target=/root/.netrc go mod download

Runtime secrets are given to the container, not the image, as environment variables or files.

Practice Version and commit

Declare var version = "dev" and var commit = "yok", to be filled with ldflags. A function info() string returns version + " (" + commit + ")"; print the result.

Starter code

main.go
package main

import "fmt"

func main() {
    // buraya yaz
}

Sample solution

main.go
package main

import "fmt"

var (
    version = "dev"
    commit  = "yok"
)

func info() string {
    return version + " (" + commit + ")"
}

func main() {
    fmt.Println(info())
}

Expected output

dev (yok)

Questions and answers

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

1 What is the difference between ARG and ENV?

  • ENV exists only at build time
  • ARG stores secret values
  • ARG exists only at build time, ENV also in the running container Correct answer
  • There is no difference

Explanation: ARG values don't carry into the runtime environment; settings the program needs while running are given with ENV.

2 Build variable

Question 
____ VERSION=dev
RUN go build -ldflags "-X main.version=${VERSION}" .
Answer 
ARG VERSION=dev
RUN go build -ldflags "-X main.version=${VERSION}" .

Explanation: ARG is a build-time variable that docker build --build-arg can change.

3 Instruction and when

Term Meaning
ARG Build time
ENV Run time
LABEL Image metadata
--mount=type=secret Secret build data

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