VOL-1893 Fix voltha-openolt-adapter Makefile

Target to build docker images needs to be named docker-build for
build automation to work correctly.

Change-Id: Ib18877e5d2e0d515159e5e3fb5cb369ee27791ee
diff --git a/Makefile b/Makefile
index e4b0e10..d962531 100644
--- a/Makefile
+++ b/Makefile
@@ -56,7 +56,7 @@
 	@echo "Usage: make [<target>]"
 	@echo "where available targets are:"
 	@echo
-	@echo "build             : Build openolt adapter docker image"
+	@echo "docker-build      : Build openolt adapter docker image"
 	@echo "help              : Print this help"
 	@echo "docker-push       : Push the docker images to an external repository"
 	@echo "lint              : Run lint verification, depenancy, gofmt and reference check"
@@ -83,7 +83,7 @@
 
 ## Docker targets
 
-build: local-protos local-voltha
+docker-build: local-protos local-voltha
 	docker build $(DOCKER_BUILD_ARGS) -t ${ADAPTER_IMAGENAME} -f docker/Dockerfile.openolt .
 
 docker-push: