Simplify multi-arch building. (prometheus-operator#3035)

Default build is linux/amd64

For any other target:
OS=<os> ARCH=<arch> make

Author: NicolasFloquet

Dockerfile

@@ -1,4 +1,6 @@
-FROM quay.io/prometheus/busybox:latest
+ARG ARCH="amd64"
+ARG OS="linux"
+FROM quay.io/prometheus/busybox-${OS}-${ARCH}:latest
 
 ADD operator /bin/operator
 

Makefile

@@ -1,5 +1,8 @@
 SHELL=/bin/bash -o pipefail
 
+OS?=linux
+ARCH?=amd64
+
 GO_PKG=github.com/coreos/prometheus-operator
 REPO?=quay.io/coreos/prometheus-operator
 REPO_PROMETHEUS_CONFIG_RELOADER?=quay.io/coreos/prometheus-config-reloader
@@ -39,7 +42,7 @@ K8S_GEN_DEPS:=.header
 K8S_GEN_DEPS+=$(TYPES_V1_TARGET)
 K8S_GEN_DEPS+=$(foreach bin,$(K8S_GEN_BINARIES),$(FIRST_GOPATH)/bin/$(bin))
 
-GO_BUILD_RECIPE=GOOS=linux CGO_ENABLED=0 go build -mod=vendor -ldflags="-s -X $(GO_PKG)/pkg/version.Version=$(VERSION)"
+GO_BUILD_RECIPE=GOOS=$(OS) GOARCH=$(ARCH) CGO_ENABLED=0 go build -mod=vendor -ldflags="-s -X $(GO_PKG)/pkg/version.Version=$(VERSION)"
 pkgs = $(shell go list ./... | grep -v /vendor/ | grep -v /test/ | grep -v /contrib/)
 
 CONTAINER_CMD:=docker run --rm \
@@ -130,7 +133,7 @@ image: .hack-operator-image .hack-prometheus-config-reloader-image
 # Create empty target file, for the sole purpose of recording when this target
 # was last executed via the last-modification timestamp on the file. See
 # https://www.gnu.org/software/make/manual/make.html#Empty-Targets
-	docker build -t $(REPO):$(TAG) .
+	docker build --build-arg ARCH=$(ARCH) --build-arg OS=$(OS) -t $(REPO):$(TAG) .
 	touch $@
 
 .hack-prometheus-config-reloader-image: cmd/prometheus-config-reloader/Dockerfile prometheus-config-reloader

cmd/prometheus-config-reloader/Dockerfile

@@ -1,4 +1,6 @@
-FROM quay.io/prometheus/busybox:latest
+ARG ARCH="amd64"
+ARG OS="linux"
+FROM quay.io/prometheus/busybox-${OS}-${ARCH}:latest
 
 ADD prometheus-config-reloader /bin/prometheus-config-reloader