session_9.md

Builder images to create smaller final images

In this lab session we are going to write efficient Dockerfiles to create smaller images using builder images

Step 1 :- Write the simple Go program

In this step we are going to write simple Go program which takes run time arguments and creates an executable
Create the file main.go put below code in it and save it.

package main
import (
    "fmt"
)
func main() {
    fmt.Println("Sagar says, Hello!")
}

Step 2 :- Write the Dockerfile

In this step we are going to write the dockerfile which load the base image as a builder image, copy our contents to that image, build our Go program, then copy the executable created to the other smaller, final image and execute it.
create the file Dockerfile put below code in it and save it.

ARG CODE_VERSION=latest
FROM golang:${CODE_VERSION} as builder
WORKDIR /app
COPY . .
RUN go build main.go

FROM alpine:${CODE_VERSION}
WORKDIR /root/
COPY --from=builder /app/main .
CMD ["./main"]

The as keyword is used to provide an alias to the builder image so that files in it's filesystem can be referenced later in the Dockerfile.

--from will reference the alias of the builder image to access it's filesystem and copy the executable from the builder image's WOKRDIR to the final image's WORKDIR

Step 3 :- Build the image using dockerfile

In this step we are going to build the image using dockerfile

docker build -f Dockerfile -t demo/docker-builder:1 . 

Run docker images | grep demo/docker-builder and check the size of the image

docker-tut-builder  latest  802d8ccd4758    About a minute ago   8.14MB

But if we see the base builder image, we see that it is much, much bigger (350x)

golang              alpine   48260c3da24c   6 days ago           359MB

Step 4 :- Run the Container using our image which perform our particular task

In this step we are going to run the container using our image

docker run -it demo/docker-builder:1

We should get such an output:

$ docker run -it demo/docker-builder:1
Sagar says, Hello!