This section explores multi-stage builds. There are two main reasons for why you’d want to use multi-stage builds:
- They allow you to run build steps in parallel, making your build pipeline faster and more efficient.
- They allow you to create a final image with a smaller footprint, containing only what's needed to run your program.
In a Dockerfile, a build stage is represented by a
FROM instruction. The
Dockerfile from the previous section doesn’t leverage multi-stage builds. It’s
all one build stage. That means that the final image is bloated with resources
used to compile the program.
$ docker build --tag=buildme . $ docker images buildme REPOSITORY TAG IMAGE ID CREATED SIZE buildme latest c021c8a7051f 5 seconds ago 150MB
The program compiles to executable binaries, so you don’t need Go language utilities to exist in the final image.
Using multi-stage builds, you can choose to use different base images for your build and runtime environments. You can copy build artifacts from the build stage over to the runtime stage.
Modify the Dockerfile as follows. This change creates another stage using a
scratch image as a base. In the final
scratch stage, the binaries
built in the previous stage are copied over to the filesystem of the new stage.
# syntax=docker/dockerfile:1 FROM golang:1.20-alpine WORKDIR /src COPY go.mod go.sum . RUN go mod download COPY . . RUN go build -o /bin/client ./cmd/client RUN go build -o /bin/server ./cmd/server + + FROM scratch + COPY --from=0 /bin/client /bin/server /bin/ ENTRYPOINT [ "/bin/server" ]
Now if you build the image and inspect it, you should see a significantly smaller number:
$ docker build --tag=buildme . $ docker images buildme REPOSITORY TAG IMAGE ID CREATED SIZE buildme latest 436032454dd8 7 seconds ago 8.45MB
The image went from 150MB to only just 8.45MB in size. That’s because the resulting image only contains the binaries, and nothing else.
You've reduced the footprint of the image. The following step shows how you can improve build speed with multi-stage builds, using parallelism. The build currently produces the binaries one after the other. There is no reason why you need to build the client before building the server, or vice versa.
You can split the binary-building steps into separate stages. In the final
scratch stage, copy the binaries from each corresponding build stage. By
segmenting these builds into separate stages, Docker can run them in parallel.
The stages for building each binary both require the Go compilation tools and
application dependencies. Define these common steps as a reusable base stage.
You can do that by assigning a name to the stage using the pattern
FROM image AS stage_name. This allows you to reference the stage name in a
FROM instruction of another stage (
You can also assign a name to the binary-building stages, and reference the
stage name in the
COPY --from=stage_name instruction when copying the binaries
to the final
# syntax=docker/dockerfile:1 - FROM golang:1.20-alpine + FROM golang:1.20-alpine AS base WORKDIR /src COPY go.mod go.sum . RUN go mod download COPY . . + + FROM base AS build-client RUN go build -o /bin/client ./cmd/client + + FROM base AS build-server RUN go build -o /bin/server ./cmd/server FROM scratch - COPY --from=0 /bin/client /bin/server /bin/ + COPY --from=build-client /bin/client /bin/ + COPY --from=build-server /bin/server /bin/ ENTRYPOINT [ "/bin/server" ]
Now, instead of first building the binaries one after the other, the
build-server stages are executed simultaneously.
The final image is now small, and you’re building it efficiently using parallelism. But this image is slightly strange, in that it contains both the client and the server binary in the same image. Shouldn’t these be two different images?
It’s possible to create multiple different images using a single Dockerfile. You
can specify a target stage of a build using the
--target flag. Replace the
FROM scratch stage with two separate stages named
# syntax=docker/dockerfile:1 FROM golang:1.20-alpine AS base WORKDIR /src COPY go.mod go.sum . RUN go mod download COPY . . FROM base AS build-client RUN go build -o /bin/client ./cmd/client FROM base AS build-server RUN go build -o /bin/server ./cmd/server - FROM scratch - COPY --from=build-client /bin/client /bin/ - COPY --from=build-server /bin/server /bin/ - ENTRYPOINT [ "/bin/server" ] + FROM scratch AS client + COPY --from=build-client /bin/client /bin/ + ENTRYPOINT [ "/bin/client" ] + FROM scratch AS server + COPY --from=build-server /bin/server /bin/ + ENTRYPOINT [ "/bin/server" ]
And now you can build the client and server programs as separate Docker images (tags):
$ docker build --tag=buildme-client --target=client . $ docker build --tag=buildme-server --target=server . $ docker images buildme REPOSITORY TAG IMAGE ID CREATED SIZE buildme-client latest 659105f8e6d7 20 seconds ago 4.25MB buildme-server latest 666d492d9f13 5 seconds ago 4.2MB
The images are now even smaller, about 4 MB each.
This change also avoids having to build both binaries each time. When selecting
to build the
client target, Docker only builds the stages leading up to
that target. The
server stages are skipped if they’re not
needed. Likewise, building the
server target skips the
Multi-stage builds are useful for creating images with less bloat and a smaller footprint, and also helps to make builds run faster.
The next section describes how you can use file mounts to further improve build speeds.