Optimize cluster-autoscaler release build process

The release process for cluster-autoscaler was taking approximately
15 minutes to complete (~5 mins/arch on my machine), largely due to redundant Go dependency downloads.

This commit introduces caching optimizations:
1. Dockerfile: Reordered instructions to copy go.mod/go.sum first and added 'RUN go mod download' to cache dependencies.
2. Build Cache: Added '--mount=type=cache' to utilize Go build cache.
3. Context: Added '.dockerignore' to reduce build context size.
4. Makefile: Updated to pass 'LDFLAGS_FLAG' and 'TAGS_FLAG' as build args.

Author: Choraden

cluster-autoscaler/.dockerignore

@@ -0,0 +1,24 @@
+# Ignore git and GitHub folders
+.git
+.github
+
+# Ignore Go binaries and build artifacts
+cluster-autoscaler-*
+
+# Ignore test files
+*_test.go
+e2e/
+
+# Ignore documentation and other non-build files
+*.md
+LICENSE
+OWNERS
+OWNERS_ALIASES
+CONTRIBUTING.md
+SECURITY_CONTACTS
+code-of-conduct.md
+cloudbuild.yaml
+Makefile
+push_image.sh
+update_toc.py
+*.txt

cluster-autoscaler/Dockerfile

@@ -15,13 +15,24 @@ FROM --platform=$BUILDPLATFORM golang:1.25 AS builder
 
 WORKDIR /workspace
 
+# Copy go.mod and go.sum files first to cache dependencies
+COPY go.mod go.sum ./
+COPY apis/go.mod apis/go.sum ./apis/
+
+# Download dependencies
+RUN go mod download
+
+# Copy the rest of the source code
 COPY . .
 
 ARG GOARCH
 ARG LDFLAGS_FLAG
 ARG TAGS_FLAG
 
-RUN CGO_ENABLED=0 GOOS=linux go build -o cluster-autoscaler-$GOARCH $LDFLAGS_FLAG $TAGS_FLAG
+RUN --mount=type=cache,target=/root/.cache/go-build \
+    --mount=type=cache,target=/go/pkg/mod \
+    CGO_ENABLED=0 GOOS=linux go build -o cluster-autoscaler-$GOARCH $LDFLAGS_FLAG $TAGS_FLAG
+
 FROM gcr.io/distroless/static:nonroot
 ARG GOARCH
 COPY --from=builder /workspace/cluster-autoscaler-$GOARCH /cluster-autoscaler

cluster-autoscaler/Makefile

@@ -73,6 +73,8 @@ make-image: make-image-arch-$(GOARCH)
 make-image-arch-%:
 	GOOS=$(GOOS) docker buildx build --pull --platform linux/$* \
 		--build-arg "GOARCH=$*" \
+		--build-arg "LDFLAGS_FLAG=${LDFLAGS_FLAG}" \
+		--build-arg "TAGS_FLAG=${TAGS_FLAG}" \
 		-t ${IMAGE}-$*:${TAG} \
 		-f Dockerfile .
 	@echo "Image ${TAG}${FOR_PROVIDER}-$* completed"