Add dockerfile for ppc64le (#638)

* Add dockerfile for ppc64le and related changes

* Pass the fill file as DOCKEFILE

* Add the dockerfile name to build msg

Dockerfile.ppc64le

@@ -0,0 +1,7 @@
+FROM        ppc64le/busybox:glibc
+
+COPY node_exporter /bin/node_exporter
+
+EXPOSE      9100
+USER        nobody
+ENTRYPOINT  [ "/bin/node_exporter" ]

Makefile

@@ -22,6 +22,8 @@ PREFIX                  ?= $(shell pwd)
 BIN_DIR                 ?= $(shell pwd)
 DOCKER_IMAGE_NAME       ?= node-exporter
 DOCKER_IMAGE_TAG        ?= $(subst /,-,$(shell git rev-parse --abbrev-ref HEAD))
+MACH                    ?= $(shell uname -m)
+DOCKERFILE              ?= Dockerfile
 
 ifeq ($(OS),Windows_NT)
     OS_detected := Windows
@@ -77,8 +79,11 @@ tarball: $(PROMU)
 	@$(PROMU) tarball --prefix $(PREFIX) $(BIN_DIR)
 
 docker:
-	@echo ">> building docker image"
-	@docker build -t "$(DOCKER_IMAGE_NAME):$(DOCKER_IMAGE_TAG)" .
+ifeq ($(MACH), ppc64le)
+	$(eval DOCKERFILE=Dockerfile.ppc64le)
+endif
+	@echo ">> building docker image from $(DOCKERFILE)"
+	@docker build --file $(DOCKERFILE) -t "$(DOCKER_IMAGE_NAME):$(DOCKER_IMAGE_TAG)" .
 
 test-docker:
 	@echo ">> testing docker image"