Cross-compile for arm64 and mount go cache

Closes #29

Provides is a very efficient way to build multi-arch images:
https://www.docker.com/blog/faster-multi-platform-builds-dockerfile-cross-compilation-guide/

Dockerfile

@@ -1,11 +1,21 @@
-ARG GOARCH="amd64"
-FROM golang:1.22 AS builder
+FROM --platform=$BUILDPLATFORM golang:1.22 AS builder
+
 WORKDIR /src
+
+COPY go.mod go.sum .
+RUN --mount=type=cache,target=/go/pkg \
+    go mod download
+
 COPY . .
-# build
-RUN go mod download
-RUN CGO_ENABLED=0 go build -o /go/bin/netpol ./cmd
+
+ARG TARGETOS TARGETARCH
+RUN --mount=type=cache,target=/root/.cache/go-build \
+    --mount=type=cache,target=/go/pkg \
+    CGO_ENABLED=0 GOOS=$TARGETOS GOARCH=$TARGETARCH \
+    go build -o /go/bin/netpol ./cmd
+
 # STEP 2: Build small image
 FROM registry.k8s.io/build-image/distroless-iptables:v0.5.2
 COPY --from=builder --chown=root:root /go/bin/netpol /bin/netpol
+
 CMD ["/bin/netpol"]

Makefile

@@ -35,9 +35,16 @@ REGISTRY?=gcr.io/k8s-staging-networking
 TAG?=$(shell echo "$$(date +v%Y%m%d)-$$(git describe --always --dirty)")
 # the full image tag
 IMAGE?=$(REGISTRY)/$(IMAGE_NAME):$(TAG)
+PLATFORMS?=linux/amd64,linux/arm64
 
-# required to enable buildx
-export DOCKER_CLI_EXPERIMENTAL=enabled
 image-build:
-# docker buildx build --platform=${PLATFORMS} $(OUTPUT) --progress=$(PROGRESS) -t ${IMAGE} --pull $(EXTRA_BUILD_OPT) .
-	docker build . -t ${IMAGE}
+	docker buildx build . \
+		--platform="${PLATFORMS}" \
+		--tag="${IMAGE}" \
+		--load
+
+image-push:
+	docker buildx build . \
+		--platform="${PLATFORMS}" \
+		--tag="${IMAGE}" \
+		--push