Skip to content

Docker Images for Go

Surprisingly Docker was built with the programming language Go.

Go Application

Let's start with a simple Go webserver and init a go application:

go mod init go-webserver

Now we write the main.go file.

main.go
 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
18
19
20
21
package main

import (
    "fmt"
    "html"
    "log"
    "net/http"
)

func main() {
    http.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
        fmt.Fprintf(w, "Hello, %q", html.EscapeString(r.URL.Path))
    })

    http.HandleFunc("/hi", func(w http.ResponseWriter, r *http.Request){
        fmt.Fprintf(w, "Hi")
    })

    log.Fatal(http.ListenAndServe(":8081", nil))

}

If we run it with go run main.go and access the URL with http://localhost:8081/go we get Hello, "/go" as response.

Writing the Dockerfile

We have a working Go application, now let's start to build it as a Docker image. For that we need first a Dockerfile in the same location, where the main.go file is located.

Let's create a Dockerfile for that to build images:

Dockerfile
FROM golang:1.18.1-alpine

RUN mkdir /app

WORKDIR /app

COPY . .

RUN go build -o main .

CMD ["/app/main"]

Build the image

Now we have a Dockerfile and can build the image:

docker build . -t go-webserver:latest

Start or push the image

From that point we can start the image or push it onto an image repository.

Here is a demonstration, how to start the image:

docker run --rm -it -p 8081:8081 go-webserver

Now we can access again the browser: http://localhost:8081/go and the response should be Hello, "/go".

Smaller images

Since we compile the go application, we actually don't need the Go files anymore in the image. To save some space we can build the image in two stages (multistage).

Here is an example to do that:

Dockerfile
FROM golang:1.18.1-alpine AS BUILDER
WORKDIR /app
COPY . .
RUN go build -o app main.go

FROM alpine:latest
WORKDIR /app
COPY --from=BUILDER /app ./
CMD ["./app"]

Since we built the Go application on an alpine, we can use instead of golang:alpine just alpine, which is a smaller image.\

We built and start the image:

docker build . -t go-webserver-alpine
docker run --rm -it -p 8081:8081 go-webserver-alpine

If we compare those two images, there is a significant difference:

go-webserver-alpine            latest        4fe92b6799b2   38 seconds ago   11.4MB
go-webserver                   latest        c67bfbfc1bba   7 minutes ago    332MB