Multi-stage builds are useful to anyone who has struggled to optimize Dockerfiles while keeping them easy to read and maintain.
Use multi-stage builds#
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.
The following Dockerfile has two separate stages: one for building a binary, and another where the binary gets copied from the first stage into the next stage.
# syntax=docker/dockerfile:1
FROM golang:\{\{\% param "example_go_version" \%\}\}
WORKDIR /src
COPY <<EOF ./main.go
package main
import "fmt"
func main() {
fmt.Println("hello, world")
}
EOF
RUN go build -o /bin/hello ./main.go
FROM scratch
COPY --from=0 /bin/hello /bin/hello
CMD ["/bin/hello"]
You only need the single Dockerfile. No need for a separate build script. Just
run docker build.
$ docker build -t hello .
The end result is a tiny production image with nothing but the binary inside. None of the build tools required to build the application are included in the resulting image.
How does it work? The second FROM instruction starts a new build stage with
the scratch 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 stages#
By default, the stages aren't 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.
# syntax=docker/dockerfile:1
FROM golang:\{\{\% param "example_go_version" \%\}\} AS build
WORKDIR /src
COPY <<EOF /src/main.go
package main
import "fmt"
func main() {
fmt.Println("hello, world")
}
EOF
RUN go build -o /bin/hello ./main.go
FROM scratch
COPY --from=build /bin/hello /bin/hello
CMD ["/bin/hello"]
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 build:
$ docker build --target build -t hello .
A few scenarios where this might be useful 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 aren't 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
Use a previous stage as a new stage#
You can pick up where a previous stage left off by referring to it when using
the FROM directive. For example:
# syntax=docker/dockerfile:1
FROM alpine:latest AS builder
RUN apk --no-cache add build-base
FROM builder AS build1
COPY source1.cpp source.cpp
RUN g++ -o /binary source.cpp
FROM builder AS build2
COPY source2.cpp source.cpp
RUN g++ -o /binary source.cpp
Differences between legacy builder and BuildKit#
The legacy Docker Engine builder processes all stages of a Dockerfile leading
up to the selected --target. It will build a stage even if the selected
target doesn't depend on that stage.
BuildKit only builds the stages that the target stage depends on.
For example, given the following Dockerfile:
# syntax=docker/dockerfile:1
FROM ubuntu AS base
RUN echo "base"
FROM base AS stage1
RUN echo "stage1"
FROM base AS stage2
RUN echo "stage2"
With BuildKit enabled, building the
stage2 target in this Dockerfile means only base and stage2 are processed.
There is no dependency on stage1, so it's skipped.
$ DOCKER_BUILDKIT=1 docker build --no-cache -f Dockerfile --target stage2 .
[+] Building 0.4s (7/7) FINISHED
=> [internal] load build definition from Dockerfile 0.0s
=> => transferring dockerfile: 36B 0.0s
=> [internal] load .dockerignore 0.0s
=> => transferring context: 2B 0.0s
=> [internal] load metadata for docker.io/library/ubuntu:latest 0.0s
=> CACHED [base 1/2] FROM docker.io/library/ubuntu 0.0s
=> [base 2/2] RUN echo "base" 0.1s
=> [stage2 1/1] RUN echo "stage2" 0.2s
=> exporting to image 0.0s
=> => exporting layers 0.0s
=> => writing image sha256:f55003b607cef37614f607f0728e6fd4d113a4bf7ef12210da338c716f2cfd15 0.0s
On the other hand, building the same target without BuildKit results in all stages being processed:
$ DOCKER_BUILDKIT=0 docker build --no-cache -f Dockerfile --target stage2 .
Sending build context to Docker daemon 219.1kB
Step 1/6 : FROM ubuntu AS base
---> a7870fd478f4
Step 2/6 : RUN echo "base"
---> Running in e850d0e42eca
base
Removing intermediate container e850d0e42eca
---> d9f69f23cac8
Step 3/6 : FROM base AS stage1
---> d9f69f23cac8
Step 4/6 : RUN echo "stage1"
---> Running in 758ba6c1a9a3
stage1
Removing intermediate container 758ba6c1a9a3
---> 396baa55b8c3
Step 5/6 : FROM base AS stage2
---> d9f69f23cac8
Step 6/6 : RUN echo "stage2"
---> Running in bbc025b93175
stage2
Removing intermediate container bbc025b93175
---> 09fc3770a9c4
Successfully built 09fc3770a9c4
The legacy builder processes stage1, even if stage2 doesn't depend on it.