How to flatten a Docker image?

Question

I made a Docker container which is fairly large. When I commit the container to create an image, the image is about 7.8 GB big. But when I export the container (not save the image!) to a tarball and re-import it, the image is only 3 GB big. Of course the history is lost, but this OK for me, since the image is "done" in my opinion and ready for deployment.

How can I flatten an image/container without exporting it to the disk and importing it again? And: Is it a wise idea to do that or am I missing some important point?

Answer 1

Now that Docker has released the multi-stage builds in 17.05, you can reformat your build to look like this:

FROM buildimage as build # your existing build steps here FROM scratch COPY --from=build / / CMD ["/your/start/script"] 

The result will be your build environment layers are cached on the build server, but only a flattened copy will exist in the resulting image that you tag and push.

---

Note, you would typically reformulate this to have a complex build environment and only copy over a few directories. Here's an example with Go to make a single binary image from source code and a single build command without installing Go on the host and compiling outside of docker:

$ cat Dockerfile  ARG GOLANG_VER=1.8 FROM golang:${GOLANG_VER} as builder WORKDIR /go/src/app COPY . . RUN go-wrapper download  RUN go-wrapper install  FROM scratch COPY --from=builder /go/bin/app /app CMD ["/app"] 

The go file is a simple hello world:

$ cat hello.go  package main  import "fmt"  func main() {         fmt.Printf("Hello, world.\n") } 

The build creates both environments, the build environment and the scratch one, and then tags the scratch one:

$ docker build -t test-multi-hello .                                                                                                                               Sending build context to Docker daemon  4.096kB Step 1/9 : ARG GOLANG_VER=1.8  --->  Step 2/9 : FROM golang:${GOLANG_VER} as builder  ---> a0c61f0b0796 Step 3/9 : WORKDIR /go/src/app  ---> Using cache  ---> af5177aae437 Step 4/9 : COPY . .  ---> Using cache  ---> 976490d44468 Step 5/9 : RUN go-wrapper download  ---> Using cache  ---> e31ac3ce83c3 Step 6/9 : RUN go-wrapper install  ---> Using cache  ---> 2630f482fe78 Step 7/9 : FROM scratch  --->  Step 8/9 : COPY --from=builder /go/bin/app /app  ---> Using cache  ---> 5645db256412 Step 9/9 : CMD /app  ---> Using cache  ---> 8d428d6f7113 Successfully built 8d428d6f7113 Successfully tagged test-multi-hello:latest 

Looking at the images, only the single binary is in the image being shipped, while the build environment is over 700MB:

$ docker images | grep 2630f482fe78 <none>                <none>              2630f482fe78        6 days ago          700MB  $ docker images | grep 8d428d6f7113 test-multi-hello      latest              8d428d6f7113        6 days ago          1.56MB 

And yes, it runs:

$ docker run --rm test-multi-hello  Hello, world.