Dockerfile

Learn to write a Dockerfile for a Go app.

Dockerfile

A Dockerfile is a text file describing step by step how to build an image. The simplest Go Dockerfile:

FROM golang:1.23
WORKDIR /app
COPY . .
RUN go build -o server .
CMD ["./server"]

FROM sets the base image, WORKDIR the working folder, COPY the files, RUN the build command and CMD the command run when the container starts. docker build -t app . builds the image.

.dockerignore

COPY . . sends everything in the folder. A .dockerignore file leaves out unneeded or secret files, speeds up builds and protects the cache:

.git
*.env
bin/
node_modules/

If you use ENTRYPOINT instead of CMD, arguments given to the container are appended to your program.

Exercise Container-ready server

A server running in a container must listen on all interfaces. Write addr(port string) string returning ":" + port. Print addr("8080").

Show example solution

package main

import "fmt"

func addr(port string) string {
    return ":" + port
}

func main() {
    fmt.Println(addr("8080"))
}

Test yourself

What is the biggest problem of this single-stage image?
- Go can't compile
- It can't open a port
- The whole Go toolchain stays in the image, making it hundreds of MB
- It doesn't run
Show answer

The whole Go toolchain stays in the image, making it hundreds of MB — The golang base contains the compiler and libraries (~800 MB); running needs only the binary. Multi-stage builds solve this.

Base image

____ golang:1.23
WORKDIR /app

Show answer

FROM golang:1.23
WORKDIR /app

Instruction and role

FROM

Base image
COPY

Copies files
RUN

Build-time command
CMD

Startup command

Next lesson: Multi-stage build

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