The Multi-Stage feature allows a single Dockerfile to contain multiple stages in order to produce the desired, optimised, Docker Image.

Previously, the problem would have been solved with two Dockerfiles. One file would have the steps to build the binary and artifacts using a development container, the second would be optimised for production and not include the development tools.

By removing development tooling in the production image, you reproduce the attack surface and improve the deployment time.

Sample Code

Start by deploying a sample Golang HTTP Server. This currently using a two staged Docker Build approach. This scenario will create a new Dockerfile that allows the image to be built using a single command.

cd /home/scrapbook/tutorial && git clone https://github.com/katacoda/golang-http-server.git

Multi-Stage Dockerfile

Using the editor, create a Multi-Stage Dockerfile. The first stage using the Golang SDK to build a binary. The second stage copies the resulting binary into a optimised Docker Image.

# First Stage
FROM golang:1.6-alpine

RUN mkdir /app
ADD . /app/
WORKDIR /app
RUN CGO_ENABLED=0 GOOS=linux go build -a -installsuffix cgo -o main .

# Second Stage
FROM alpine
EXPOSE 80
CMD ["/app"]

# Copy from first stage
COPY --from=0 /app/main /app

There are currently discussions about improving the syntax that you can follow at https://github.com/docker/docker/pull/31257

With the new syntax for the Dockerfile in place, the build process is identical to previously.

Task

Create the desired Docker Image using the build command below.

docker build -f Dockerfile.multi -t golang-app .

The result will be two images. One untagged that was used for the first stage and the second, smaller image, our target image.

docker images

If you receive the error, COPY --from=0 /build/out /app/ Unknown flag: from, it means you're running an older version of Docker without the multi-stage support. Step 1 of this scenario upgrades the current Docker version.

The image can be launched and deployed without any changes required.

docker run -d -p 80:80 golang-app

curl localhost