Reduce Docker Image Size by Using Multi-stage Builds

If you don’t take any optimization measures, docker images can easily get large. And in most cases we just wrapped too many inessential things into the images. So, we should take actions to get rid of it.
Let’s take a look at a common example Dockerfile for building an application written in Go.

FROM golang:1.10.2-stretch

WORKDIR /go/src/github.com/ggicci/docker-example-server
COPY . .

RUN go install

LABEL \
 me.ggicci.appdemo.image.created="2006-01-02T15:04:05+08:00" \
 me.ggicci.appdemo.image.version="1.0.0"

# (and more)

WORKDIR /app
ENTRYPOINT [ "docker-example-server" ]
EXPOSE 8080

After building the image by running command docker build -t example:1.0.0 ., we can see the size of the image we get is 800+ MB.

REPOSITORY          TAG                 IMAGE ID            CREATED             SIZE
example             1.0.0               9daf905d097d        12 seconds ago      801MB

However as we know, go programs are statically compiled and linked. We don’t need many things in the image to run the final binary:

1. Source code.
2. Go compiling tools (the go command).
3. Big linux image.

Then, let’s cut them off.

Use Multi-stage Builds

# Stage: builder
FROM golang:1.10.2-stretch as builder

WORKDIR /go/src/github.com/ggicci/docker-example-server
COPY . .

RUN go install

# Stage: runner
FROM alpine:3.7

WORKDIR /app
COPY --from=builder /go/bin/docker-example-server /app

LABEL \
  me.ggicci.appdemo.image.created="2006-01-02T15:04:05+08:00" \
  me.ggicci.appdemo.image.version="1.0.0"
  # (and more)

ENTRYPOINT [ "docker-example-server" ]
EXPOSE 8080

There are two stages defined in the Dockerfile above:

• builder: based on a linux image with go tools installed, we build our application;
• runner: based on a tiny linux image alpine, we copy the application binary from the builder image and discard the builder image.

Now, the image we get is small enough. Which is only 10+ MB.

REPOSITORY          TAG                 IMAGE ID            CREATED             SIZE
example-2           1.0.0               77488a7264a1        9 seconds ago       11MB

Cheers 🎉