This is a multi-stage build. This is used to keep the running docker container small while still be able to build/compile things needing a lot of dependencies.
For example a go application could be built by using:
FROM golang:1.7.3 AS builder
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 .
CMD ["./app"]
So in the first part we need a complete go environment to compile our software. Notice the name for the first part and the alias builder
FROM golang:1.7.3 AS builder
In the second part beginning from the second FROM we only need the compiled app and no other go dependencies anymore. So we can change the base image to using a much smaller alpine Linux.
But the compiled files are still in our builder image and not part of the image we want to start.
So we need to copy files from the builder image via
COPY --from=builder
You can have as many stages as you want. The last one is the one defining the image which will be the template for the docker container.
You can read more about it in the official documentation:
https://docs.docker.com/develop/develop-images/multistage-build/