Fix bare 'item' in build-container-image

Move the setting of the containerfile into the loop (thus in build.yaml)
to follow the semantics from roles/build-container-image/common.rst for
container_filename and container_images.container_filename.

Co-Authored-By: Andreas Jaeger <aj@suse.com>
Change-Id: Id39f13cc666e9f99557447073cc0d362991f3506

roles/build-container-image/tasks/build.yaml

@@ -25,6 +25,10 @@
     loop_var: zj_sibling
   when: item.siblings is defined
 
+- name: Set container filename arg
+  set_fact:
+    containerfile: "{{ item.container_filename|default(container_filename)|default('') }}"
+
 - name: Build a container image
   command: >-
     {{ container_command }} build {{ item.path | default('.') }} {% if containerfile %}-f {{ containerfile }}{% endif %}

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

@@ -14,10 +14,6 @@
     - result_json_stat.stat.size > 0
     - "'buildset_registry' in (lookup('file', zuul.executor.work_root + '/results.json') | from_json)"
 
-- name: Set container filename arg
-  set_fact:
-    containerfile: "{{ item.container_filename|default(container_filename|default('')) }}"
-
 - name: Build container images
   include_tasks: build.yaml
   loop: "{{ container_images }}"