podman buildset testing: dump image list

Explictly dump the image list for easier debugging, and add a little
note about what's going on.

Change-Id: I27b3003a63d549c48a9326c807a7f14b97ba1dad

playbooks/functional-test/podman-buildset.yaml

@@ -61,7 +61,19 @@
 - name: Print list of images
   command: podman image ls --all --digests --no-trunc
   register: image_list
-  failed_when: "'docker.io/test/image' not in image_list.stdout"
+
+# Pulling the short-name test/image should result in a pull of
+# "docker.io/test/image" from the localhost:9000 mirror thanks to
+# unqualified-serach-registries being set to "docker.io" in
+# /etc/containers/registries.conf
+- name: Show images
+  debug:
+    var: image_list.stdout
+
+- name: Check for test image
+  fail:
+    msg: 'docker.io/test/image not found in podman image list'
+  when: "'docker.io/test/image' not in image_list.stdout"
 
 - name: Remove the test image from the local cache
   command: podman rmi docker.io/test/image