added local as default for registry (#3876)

build/Makefile

@@ -99,6 +99,7 @@ common_mounts = -v $(build_path)/.config/gcloud:/root/.config/gcloud \
 				-v $(build_path)/.gomod:/go/pkg/mod
 
 # Its possible for this to change in the future, so it gets its own variable.
+REGISTRY ?= local
 workdir_path = $(mount_path)
 
 build_version = $(call sha,$(build_path)/build-image/Dockerfile)