Add root-level makefile to docker directory

README.md

@@ -19,7 +19,7 @@ ska/tango-starter      | Example image that demonstrates how to package the Star
 To build the images, from the root of this repository execute:
 
     cd docker
-    make 
+    make build
     
 ## Launching TANGO applications and services
 The docker-compose directory contains a set of files that can be used to run a

docker/Makefile

+.PHONY: tango help
+.DEFAULT_GOAL := help
+
+build:  ## build all SKA TANGO docker images
+	$(MAKE) -C tango build
+
+help:  ## show this help.
+	@grep -E '^[a-zA-Z_-]+:.*?## .*$$' $(MAKEFILE_LIST) | sort | awk 'BEGIN {FS = ":.*?## "}; {printf "\033[36m%-30s\033[0m %s\n", $$1, $$2}'