support multi-arch builds (#6, thanks @oyvindh)

Dockerfile

@@ -1,4 +1,4 @@
-FROM golang:1.15.7-alpine AS build_deps
+FROM --platform=$BUILDPLATFORM golang:1.15.7-alpine AS build_deps
 
 RUN apk add --no-cache git
 
@@ -13,8 +13,9 @@ RUN go mod download
 FROM build_deps AS build
 
 COPY . .
-
-RUN CGO_ENABLED=0 go build -o webhook -ldflags '-w -extldflags "-static"' .
+ARG TARGETOS
+ARG TARGETARCH
+RUN GOOS=$TARGETOS GOARCH=$TARGETARCH CGO_ENABLED=0 go build -o webhook -ldflags '-w -extldflags "-static"' .
 
 FROM alpine:3.9
 

Makefile

@@ -35,6 +35,15 @@ docker:
 		--build-arg "GIT_BRANCH=$(GIT_BRANCH)" \
 		-t "$(IMAGE_NAME):$(IMAGE_TAG)" .
 
+docker-push:
+	docker buildx build \
+		--push \
+		--build-arg "BUILD_DATE=$(BUILD_DATE)" \
+		--build-arg "GIT_COMMIT=$(GIT_COMMIT)" \
+		--build-arg "GIT_BRANCH=$(GIT_BRANCH)" \
+		--platform linux/amd64,linux/arm64,linux/arm/v7 \
+		-t "$(IMAGE_NAME):$(IMAGE_TAG)" .
+
 .PHONY: rendered-manifest.yaml
 rendered-manifest.yaml:
 	helm template \