Use variables for Makefile tagging

Makefile

@@ -1,12 +1,14 @@
 # Makefile
 
+SHARELATEX_BASE_TAG := sharelatex/sharelatex-base
+SHARELATEX_TAG := sharelatex/sharelatex
 
 build-base:
-	docker build -f Dockerfile-base -t sharelatex/sharelatex-base .
+	docker build -f Dockerfile-base -t  $(SHARELATEX_BASE_TAG) .
 
 
 build-community:
-	docker build -f Dockerfile -t sharelatex/sharelatex .
+	docker build -f Dockerfile -t $(SHARELATEX_TAG) .
 
 
 PHONY: build-base build-community