Makefile: use $USER/image when depending on a generated image

Te make scripts will generate docker images like
"$username/foobar-test". When depending on an previous image,
the $username must match or the build will about with image not found.

Change-Id: Ied42c3e1de9a2ffaca22ba4cd02e6a398e48e97d

m3ua-test/Dockerfile

@@ -1,4 +1,5 @@
-FROM laforge/sigtran-tests
+ARG USER=osmocom-build
+FROM $USER/sigtran-tests
 
 MAINTAINER Harald Welte <laforge@gnumonks.org>
 

make/Makefile

@@ -40,7 +40,7 @@ post-push:
 
 
 docker-build: .release
-	docker build -t $(IMAGE):latest .
+	docker build --build-arg USER=$(USERNAME) -t $(IMAGE):latest .
 	@DOCKER_MAJOR=$(shell docker -v | sed -e 's/.*version //' -e 's/,.*//' | cut -d\. -f1) ; \
 	DOCKER_MINOR=$(shell docker -v | sed -e 's/.*version //' -e 's/,.*//' | cut -d\. -f2) ; \
 

sua-test/Dockerfile

@@ -1,4 +1,5 @@
-FROM laforge/sigtran-tests
+ARG USER=osmocom-build
+FROM $USER/sigtran-tests
 
 MAINTAINER Harald Welte <laforge@gnumonks.org>