Docker multistage build
Code: Yes Created: Mar 6, 2021 5:43 PM Materials: https://docs.docker.com/develop/develop-images/multistage-build/ Reviewed: No context: docker
Multi stage build
used mainly in production
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
FROM golang:1.7.3
WORKDIR /go/src/github.com/alexellis/href-counter/
RUN go get -d -v golang.org/x/net/html
COPY app.go .
RUN CGO_ENABLED=0 GOOS=linux go build -a -installsuffix cgo -o app .
FROM alpine:latest
RUN apk --no-cache add ca-certificates
WORKDIR /root/
COPY --from=0 /go/src/github.com/alexellis/href-counter/app .
CMD ["./app"]
$ docker build -t alexellis2/href-counter:latest .
How does it work? The second FROM instruction starts a new build stage with the alpine:latest image as its base. The COPY --from=0 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
Name your build stage
By default, the stages are not named, and you refer to them by their integer number, starting with 0 for the first FROM instruction. However, you can name your stages, by adding an AS <NAME> to the FROM instruction. This example improves the previous one by naming the stages and using the name in the COPY instruction. This means that even if the instructions in your Dockerfile are re-ordered later, the COPY doesn’t break.
FROM golang:1.7.3 AS builder # the naming here is the improvement
WORKDIR /go/src/github.com/alexellis/href-counter/
RUN go get -d -v golang.org/x/net/html
COPY app.go .
RUN CGO_ENABLED=0 GOOS=linux go build -a -installsuffix cgo -o app .
FROM alpine:latest
RUN apk --no-cache add ca-certificates
WORKDIR /root/
COPY --from=builder /go/src/github.com/alexellis/href-counter/app . # copying from the old build which we gave a name
CMD ["./app"]
Stop at a specific build stage
When you build your image, you don’t necessarily need to build the entire Dockerfile including every stage. You can specify a target build stage. The following command assumes you are using the previous Dockerfile but stops at the stage named builder:
$ docker build --target builder -t alexellis2/href-counter:latest .
A few scenarios where this might be very powerful are:
- Debugging a specific build stage
- Using a
debugstage with all debugging symbols or tools enabled, and a leanproductionstage - Using a
testingstage in which your app gets populated with test data, but building for production using a different stage which uses real data
Use an external image as a “stage”🔗
When using multi-stage builds, you are not limited to copying from stages you created earlier in your Dockerfile. You can use the COPY --from instruction to copy from a separate image, either using the local image name, a tag available locally or on a Docker registry, or a tag ID. The Docker client pulls the image if necessary and copies the artifact from there. The syntax is:
COPY --from=nginx:latest /etc/nginx/nginx.conf /nginx.conf