refactor(flagd): update build.Dockerfile with buildkit caching (#724)

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## This PR
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updates `flagd/build.Dockerfile` with changes:

- use less layers (from 7 to 2)
- leverage buildkit features: cache (go mod, go build) and bind

### Related Issues
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### Notes
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- This change is motivated from the new command of docker: `docker init`
(reference:
https://docs.docker.com/engine/reference/commandline/init/#example-of-selecting-go)

### Follow-up Tasks
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noted under this section -->
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### How to test
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```
$ docker build . -f ./flagd/build.Dockerfile -t flagd
[+] Building 17.3s (12/12) FINISHED                                                                                                               docker:orbstack
 => [internal] load .dockerignore                                                                                                                            0.0s
 => => transferring context: 2B                                                                                                                              0.0s
 => [internal] load build definition from build.Dockerfile                                                                                                   0.0s
 => => transferring dockerfile: 1.74kB                                                                                                                       0.0s
 => [internal] load metadata for gcr.io/distroless/static:nonroot                                                                                            0.5s
 => [internal] load metadata for docker.io/library/golang:1.20-alpine                                                                                        1.2s
 => [builder 1/4] FROM docker.io/library/golang:1.20-alpine@sha256:fd9d9d7194ec40a9a6ae89fcaef3e47c47de7746dd5848ab5343695dbbd09f8c                          0.0s
 => [internal] load build context                                                                                                                            0.0s
 => => transferring context: 8.07kB                                                                                                                          0.0s
 => CACHED [stage-1 1/3] FROM gcr.io/distroless/static:nonroot@sha256:9ecc53c269509f63c69a266168e4a687c7eb8c0cfd753bd8bfcaa4f58a90876f                       0.0s
 => CACHED [builder 2/4] WORKDIR /src                                                                                                                        0.0s
 => CACHED [builder 3/4] RUN --mount=type=cache,target=/go/pkg/mod/     --mount=type=bind,source=./core/go.mod,target=./core/go.mod     --mount=type=bind,s  0.0s
 => [builder 4/4] RUN --mount=type=cache,target=/go/pkg/mod/     --mount=type=cache,target=/root/.cache/go-build     --mount=type=bind,source=./core,targe  15.9s
 => [stage-1 2/3] COPY --from=builder /bin/flagd-build .                                                                                                     0.0s
 => exporting to image                                                                                                                                       0.1s
 => => exporting layers                                                                                                                                      0.1s
 => => writing image sha256:ccfeaee26cdeb83fcdd70408d372f9dc382d028eb53773f062f079bdb6ff4a40                                                                 0.0s
 => => naming to docker.io/library/flagd                                                                                                                     0.0s

$ docker run flagd version

                 ______   __       ________   _______    ______      
                /_____/\ /_/\     /_______/\ /______/\  /_____/\     
                \::::_\/_\:\ \    \::: _  \ \\::::__\/__\:::_ \ \    
                 \:\/___/\\:\ \    \::(_)  \ \\:\ /____/\\:\ \ \ \   
                  \:::._\/ \:\ \____\:: __  \ \\:\\_  _\/ \:\ \ \ \  
                   \:\ \    \:\/___/\\:.\ \  \ \\:\_\ \ \  \:\/.:| | 
                    \_\/     \_____\/ \__\/\__\/ \_____\/   \____/_/     
```

Signed-off-by: Lam Tran <lam.tran@spenmo.com>
Co-authored-by: Lam Tran <lam.tran@spenmo.com>
Co-authored-by: Todd Baert <toddbaert@gmail.com>

flagd/build.Dockerfile

@@ -2,33 +2,40 @@
 # Build the manager binary
 FROM --platform=$BUILDPLATFORM golang:1.20-alpine AS builder
 
-WORKDIR /workspace
+WORKDIR /src
+
 ARG TARGETOS
 ARG TARGETARCH
 ARG VERSION
 ARG COMMIT
 ARG DATE
 
-# Copy source code
-COPY flagd/ flagd
-COPY core/ core
-
-# Setup go workspace 
-RUN go work init
-RUN go work use ./flagd
-RUN go work use ./core
-
-# Go get dependencies
-RUN cd flagd && go mod download
+# Download dependencies as a separate step to take advantage of Docker's caching.
+# Leverage a cache mount to /go/pkg/mod/ to speed up subsequent builds.
+# Leverage bind mounts to go.sum and go.mod to avoid having to copy them into
+# the container.
+RUN --mount=type=cache,target=/go/pkg/mod/ \
+    --mount=type=bind,source=./core/go.mod,target=./core/go.mod \
+    --mount=type=bind,source=./core/go.sum,target=./core/go.sum \
+    --mount=type=bind,source=./flagd/go.mod,target=./flagd/go.mod \
+    --mount=type=bind,source=./flagd/go.sum,target=./flagd/go.sum \
+    go work init ./core ./flagd && go mod download
 
-# # Build
-RUN CGO_ENABLED=0 GOOS=${TARGETOS} GOARCH=${TARGETARCH} go build -a -ldflags "-X main.version=${VERSION} -X main.commit=${COMMIT} -X main.date=${DATE}" -o flagd-build flagd/main.go
+# Build the application.
+# Leverage a cache mount to /go/pkg/mod/ to speed up subsequent builds.
+# Leverage a bind mount to the current directory to avoid having to copy the
+# source code into the container.
+RUN --mount=type=cache,target=/go/pkg/mod/ \
+    --mount=type=cache,target=/root/.cache/go-build \
+    --mount=type=bind,source=./core,target=./core \
+    --mount=type=bind,source=./flagd,target=./flagd \
+    CGO_ENABLED=0 GOOS=${TARGETOS} GOARCH=${TARGETARCH} go build -a -ldflags "-X main.version=${VERSION} -X main.commit=${COMMIT} -X main.date=${DATE}" -o /bin/flagd-build flagd/main.go
 
 # # Use distroless as minimal base image to package the manager binary
 # # Refer to https://github.com/GoogleContainerTools/distroless for more details
 FROM gcr.io/distroless/static:nonroot
 WORKDIR /
-COPY --from=builder /workspace/flagd-build .
+COPY --from=builder /bin/flagd-build .
 USER 65532:65532
 
 ENTRYPOINT ["/flagd-build"]