Docker: Multi-stage builds

Multi-stage builds is a new feature in Docker 17.05, and they will be exciting to anyone who has struggled to optimise Dockerfiles while keeping them easy to read and maintain.

One of the most challenging things about building images is keeping the image size down. Each instruction in the Dockerfile adds a layer to the image, and you need to remember to clean up any artifacts you don’t need before moving on to the next layer. To write a really efficient Dockerfile, you have traditionally needed to employ shell tricks and other logic to keep the layers as small as possible and to ensure that each layer has the artifacts it needs from the previous layer and nothing else.

This is how we used to do...

This is a simple go app to print "Hello Docker!".

[root@node1 multistage]# cat app.go 
package main

import "fmt"

func main() {  
fmt.Println("Hello Docker!")  
}

And heres our Dockerfile to build and run this application.

[root@node1 multistage]# cat Dockerfile 
FROM golang:1.7.3  
COPY app.go .  
RUN go build -o app app.go  
CMD ["./app"]  
root@ubuntu:~/multistage#  

Lets build this

[root@node1 multistage]# docker build . -t mygoapp
Sending build context to Docker daemon 3.072kBStep 1/4 : FROM golang:1.7.3  
1.7.3: Pulling from library/golang  
386a066cd84a: Pull complete  
75ea84187083: Pull complete  
88b459c9f665: Pull complete  
a31e17eb9485: Pull complete  
457559cc1d69: Pull complete  
47fe51a74a06: Pull complete  
08dacccac43c: Pull complete  
Digest: sha256:340212e9c5d062f3bfe58ff02768da70234ea734bd022a357ee6be2a6d963505Status: Downloaded newer image for golang:1.7.3  
---> ef15416724f6Step 2/4 : COPY app.go .
---> c50e64d59b04
Removing intermediate container 8198404914c9  
Step 3/4 : RUN go build -o app app.go  
---> Running in 685898ecf74f
---> 739f7b2a47c7
Removing intermediate container 685898ecf74f  
Step 4/4 : CMD ./app  
---> Running in 66334e6ca5d8
---> ab1152b774ee
Removing intermediate container 66334e6ca5d8  
Successfully built ab1152b774ee  
Successfully tagged mygoapp:latest  
[root@node1 multistage]#

[root@node1 multistage]# docker run mygoapp
Hello Docker!  
[root@node1 multistage]#

Check the size of image, its 674MB

[root@node1 multistage]# docker image ls
REPOSITORY          TAG                 IMAGE ID            CREATED             SIZE  
mygoapp             latest              efdc2b679aa6        15 seconds ago      674MB  
golang              1.7.3               ef15416724f6        8 months ago        672MB  
[root@node1 multistage]#
This is how we would love to do...

With multi-stage builds, you use multiple FROM statements in your Dockerfile. Each FROM instruction can use a different base, and each of them begins a new stage of the build. You can selectively copy artifacts from one stage to another, leaving behind everything you don’t want in the final image. To show how this works, Let’s adapt the Dockerfile from the previous section to use multi-stage builds.

[root@node1 multistage]# cat Dockerfile.multi
FROM golang:1.7.3 AS builder  
COPY app.go .  
RUN go build -o app app.go

FROM alpine:latest  
COPY --from=builder /go/app .  
CMD ["./app"]  

You only need the single Dockerfile. You don’t need a separate build script, either. Just run docker build.

[root@node1 multistage]# docker build . -f Dockerfile.multi -t mygoapp:multi
Sending build context to Docker daemon  4.096kB  
Step 1/6 : FROM golang:1.7.3 AS builder  
 ---> ef15416724f6
Step 2/6 : COPY app.go .  
 ---> Using cache
 ---> 15f89542f308
Step 3/6 : RUN go build -o app app.go  
 ---> Using cache
 ---> 09d91ef7f1a1
Step 4/6 : FROM alpine:latest  
latest: Pulling from library/alpine  
88286f41530e: Pull complete  
Digest: sha256:1072e499f3f655a032e88542330cf75b02e7bdf673278f701d7ba61629ee3ebe  
Status: Downloaded newer image for alpine:latest  
 ---> 7328f6f8b418
Step 5/6 : COPY --from=builder /go/app .  
 ---> adee51a03a34
Removing intermediate container de57adb9658f  
Step 6/6 : CMD ./app  
 ---> Running in 42c17df73436
 ---> 40b371535a83
Removing intermediate container 42c17df73436  
Successfully built 40b371535a83  
Successfully tagged mygoapp:multi  
[root@node1 multistage]#

The second FROM instruction starts a new build stage with the alpine:latest image as its base. The COPY --from=builder line copies just the built artifact from the previous stage into this new stage. The Go SDK and any intermediate artifacts are left behind, and not saved in the final image. The end result is a tiny production image.

[root@node1 multistage]# docker image ls
REPOSITORY          TAG                 IMAGE ID            CREATED              SIZE  
mygoapp             multi               40b371535a83        About a minute ago   5.6MB  
mygoapp             latest              efdc2b679aa6        5 minutes ago        674MB  
alpine              latest              7328f6f8b418        3 weeks ago          3.97MB  
golang              1.7.3               ef15416724f6        8 months ago         672MB  
[root@node1 multistage]#

Notice the difference in size. :)


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Swapnil Jain

RHCA Level X, RHCI ♦ Solution Architect ♦ DevOps Trainer & Consultant