perf(docker): use native cross-compilation instead of QEMU

Multi-arch builds were slow because Docker buildx ran the entire
builder stage under QEMU emulation for non-host architectures.
The arm64 go build took ~10 minutes on amd64 runners vs ~2.5 minutes
natively.

- Add --platform=$BUILDPLATFORM to builder FROM in Dockerfile
- Simplify docker-buildx Makefile target (remove Dockerfile.cross
  sed workaround that existed for this exact purpose)
- Drop s390x and ppc64le from PLATFORMS (only amd64/arm64 needed)

Go's cross-compiler handles GOARCH natively with CGO_ENABLED=0,
so QEMU emulation is no longer used for any RUN step. Expected
CI build time reduction from ~15 min to ~5 min.

Author: fernando-villalba

Dockerfile

@@ -3,7 +3,7 @@
 # Github workflow step anchore/scan-action scans only the final image
 # sync this intermediate FROM reference with:
 #   scan-intermediate-image.yaml
-FROM golang:1.25.7-alpine3.23 AS builder
+FROM --platform=$BUILDPLATFORM golang:1.25.7-alpine3.23 AS builder
 
 ARG TARGETOS
 ARG TARGETARCH
@@ -19,11 +19,9 @@ RUN go mod download
 # Copy the Go source (relies on .dockerignore to filter)
 COPY . .
 
-# Build
-# the GOARCH has no default value to allow the binary to be built according to the host where the command
-# was called. For example, if we call make docker-build in a local env which has the Apple Silicon M1 SO
-# the docker BUILDPLATFORM arg will be linux/arm64 when for Apple x86 it will be linux/amd64. Therefore,
-# by leaving it empty we can ensure that the container and binary shipped on it will have the same platform.
+# Build using Go's native cross-compiler (CGO_ENABLED=0).
+# --platform=$BUILDPLATFORM on the FROM makes this stage run on the host arch
+# (amd64) even when targeting arm64, avoiding slow QEMU emulation.
 RUN CGO_ENABLED=0 \
 	GOOS=${TARGETOS:-linux} \
 	GOARCH=${TARGETARCH} \

Makefile

@@ -266,9 +266,8 @@ build: manifests generate fmt vet ## Build manager binary with version metadata
 run: manifests generate fmt vet ## Run a controller from your host.
 	go run ./cmd/multigres-operator/main.go
 
-# If you wish to build the manager image targeting other platforms you can use the --platform flag.
-# (i.e. docker build --platform linux/arm64). However, you must enable docker buildKit for it.
-# More info: https://docs.docker.com/develop/develop-images/build_enhancements/
+# Cross-platform builds are handled natively via --platform=$BUILDPLATFORM in the Dockerfile.
+# Use docker-buildx target or pass --platform to docker build for multi-arch images.
 .PHONY: container
 container: ## Build container image
 	$(CONTAINER_TOOL) build -t ${IMG} .
@@ -281,22 +280,14 @@ minikube-load:
 container-push: ## Push container image
 	$(CONTAINER_TOOL) push ${IMG}
 
-# PLATFORMS defines the target platforms for the manager image be built to provide support to multiple
-# architectures. (i.e. make docker-buildx IMG=myregistry/mypoperator:0.0.1). To use this option you need to:
-# - be able to use docker buildx. More info: https://docs.docker.com/build/buildx/
-# - have enabled BuildKit. More info: https://docs.docker.com/develop/develop-images/build_enhancements/
-# - be able to push the image to your registry (i.e. if you do not set a valid value via IMG=<myregistry/image:<tag>> then the export will fail)
-# To adequately provide solutions that are compatible with multiple platforms, you should consider using this option.
-PLATFORMS ?= linux/arm64,linux/amd64,linux/s390x,linux/ppc64le
+# PLATFORMS defines the target platforms for the manager image.
+PLATFORMS ?= linux/arm64,linux/amd64
 .PHONY: docker-buildx
-docker-buildx: ## Build and push docker image for the manager for cross-platform support
-	# copy existing Dockerfile and insert --platform=${BUILDPLATFORM} into Dockerfile.cross, and preserve the original Dockerfile
-	sed -e '1 s/\(^FROM\)/FROM --platform=\$$\{BUILDPLATFORM\}/; t' -e ' 1,// s//FROM --platform=\$$\{BUILDPLATFORM\}/' Dockerfile > Dockerfile.cross
+docker-buildx: ## Build and push docker image for cross-platform support
 	- $(CONTAINER_TOOL) buildx create --name multigres-operator-builder
 	$(CONTAINER_TOOL) buildx use multigres-operator-builder
-	- $(CONTAINER_TOOL) buildx build --push --platform=$(PLATFORMS) --tag ${IMG} -f Dockerfile.cross .
+	- $(CONTAINER_TOOL) buildx build --push --platform=$(PLATFORMS) --tag ${IMG} .
 	- $(CONTAINER_TOOL) buildx rm multigres-operator-builder
-	rm Dockerfile.cross
 
 .PHONY: build-installer
 build-installer: manifests generate kustomize ## Generate a consolidated YAML with CRDs and deployment.