Clean up BASEIMAGE handling

Makefile

@@ -41,17 +41,16 @@ ARCH := $(if $(GOARCH),$(GOARCH),$(shell go env GOARCH))
 
 # Set default base image dynamically for each arch
 # TODO: make these all consistent and tagged.
-ifeq ($(ARCH),amd64)
+ifeq ($(OS)/$(ARCH),linux/amd64)
     BASEIMAGE ?= alpine:3.8
-endif
-ifeq ($(ARCH),arm)
+else ifeq ($(OS)/$(ARCH),linux/arm)
     BASEIMAGE ?= armel/busybox
-endif
-ifeq ($(ARCH),arm64)
+else ifeq ($(OS)/$(ARCH),linux/arm64)
     BASEIMAGE ?= aarch64/busybox
-endif
-ifeq ($(ARCH),ppc64le)
+else ifeq ($(OS)/$(ARCH),linux/ppc64le)
     BASEIMAGE ?= ppc64le/busybox
+else
+    $(error Unsupported target platform '$(OS)/$(ARCH)')
 endif
 
 IMAGE := $(REGISTRY)/$(BIN)