Modify Dockerfile to support multi-platform builds.

These changes enable building kubemacpool container images for
multiple platforms (amd64, s390x, arm64) from a single Dockerfile.

Signed-off-by: Ashok Pariya ashok.pariya@ibm.com

build/Dockerfile

@@ -1,4 +1,31 @@
+ARG BUILD_ARCH=amd64
+FROM --platform=linux/${BUILD_ARCH} quay.io/centos/centos:stream9 AS builder
+ARG TARGETOS
+ARG TARGETARCH
+ENV TARGETOS=${TARGETOS:-linux}
+ENV TARGETARCH=${TARGETARCH:-amd64}
+
+ARG BUILDOS
+ARG BUILDARCH
+ENV BUILDOS=${BUILDOS:-linux}
+ENV BUILDARCH=${BUILDARCH:-amd64}
+
+WORKDIR /go/src/kubemacpool
+RUN dnf install -y wget && dnf clean all
+COPY go.mod .
+COPY go.sum .
+RUN GO_VERSION=$(sed -En 's/^go +(.*)$/\1/p' go.mod) && \
+    wget https://dl.google.com/go/go${GO_VERSION}.${BUILDOS}-${BUILDARCH}.tar.gz && \
+    tar -C /usr/local -xzf go${GO_VERSION}.${BUILDOS}-${BUILDARCH}.tar.gz && \
+    rm go${GO_VERSION}.${BUILDOS}-${BUILDARCH}.tar.gz
+
+ENV PATH=$PATH:/usr/local/go/bin
+RUN go mod download
+COPY . .
+
+RUN CGO_ENABLED=0 GOOS=${TARGETOS:-linux} GOARCH=${TARGETARCH} go build -o build/_output/bin/manager github.com/k8snetworkplumbingwg/kubemacpool/cmd/manager
+
 # Copy the controller-manager into a thin image
-FROM registry.access.redhat.com/ubi9/ubi-minimal
-COPY _output/bin/manager /
+FROM --platform=linux/${TARGETARCH} registry.access.redhat.com/ubi9/ubi-minimal
+COPY --from=builder /go/src/kubemacpool/build/_output/bin/manager /
 ENTRYPOINT ["/manager"]