Pull target from item in build image

In the image building loop, the loop variable is 'item' - which is
where target is located.

Change-Id: I4c9e612f58c1fd92a32da1888af195f17cb821eb

roles/build-docker-image/tasks/main.yaml

@@ -1,8 +1,8 @@
 - name: Build a docker image
   command: >-
     docker build {{ item.path | default('.') }} -f Dockerfile
-    {% if target | default(false) -%}
-      --target {{ target }}
+    {% if item.target | default(false) -%}
+      --target {{ item.target }}
     {% endif -%}
     {% for build_arg in item.build_args | default([]) -%}
       --build-arg {{ build_arg }}