Multi stage docker build for go

Support for multistage docker build has landed in Docker earlier this year. Multi stage builds simplify the image building and the genereated images are much smaller in the size. The code/instructions are available at go-multi-stage-docker.

1. Here is our lovely go program that we want to run in the image:

   package main
   
   import "fmt"
   
   func main() {
       fmt.Println("Hello Multistage Docker builds!")
   }
   

2. Here is the Dockerfile that you can use:

   # gobase layer
   FROM golang:alpine AS go-base
   ADD . /src
   RUN cd /src && go build -o hello
   
   # final layer
   FROM alpine
   COPY --from=go-base /src/hello /app/
   ENTRYPOINT /app/hello

   Here we create two layers:

   • go-base layer: Includes go runtime. We are mounting src directory in the image and compiling our go package(hello.go) and generating the output binary hello.
   • app layer: Extends the alpine image by copying our binary(/src/hello) from the go-base layer to /app/. It also defines entrypoint for the image as our binary(/app/hello)

3. Lets build our image and name it go-multi-stage-docker :

   $ docker build -f Dockerfile -t go-multi-stage-docker:latest .

   You can see the new image would be only few MBs in the size :

   $ docker image ls go-multi-stage-docker
   
   REPOSITORY              TAG                 IMAGE ID            CREATED             SIZE
   go-multi-stage-docker   latest              5f037e697b51        4 minutes ago       5.52MB

4. Lets run our app/image

   $ docker run --rm go-multi-stage-docker
   
   Hello Multistage Docker builds!

Thats it !