os_images: skip build checks for download-only images

galaxy.yml

@@ -1,7 +1,7 @@
 ---
 namespace: stackhpc
 name: openstack
-version: 0.9.0
+version: 0.9.1
 readme: README.md
 authors:
   - StackHPC Ltd

roles/os_images/tasks/prechecks.yml

@@ -1,11 +1,14 @@
 ---
 # Here a quick check for the presence of Docker (or an alias for Podman)
 # Display a warning if not found
 - name: Check if container engine is installed
   ansible.builtin.command: which docker
   changed_when: false
   ignore_errors: true
   register: result
+  when:
+    - os_images_build | bool
+    - os_images_list | selectattr('elements', 'defined') | list | length > 0
 
 - name: Display warning message about the container engine
   ansible.builtin.assert:
@@ -14,3 +17,6 @@
     fail_msg: >
       Container runtime engine could not be found - make sure it is installed.
       Elements that depend on it will likely fail when building.
+  when:
+    - os_images_build | bool
+    - os_images_list | selectattr('elements', 'defined') | list | length > 0

roles/os_images/tasks/prereqs.yml

@@ -55,7 +55,10 @@
   # Using --reset means that we always change something
   changed_when: true
   become: true
-  when: os_images_list |
-        selectattr("architecture", "defined") |
-        rejectattr("architecture", "equalto", ansible_facts.architecture) |
-        length > 0
+  when:
+    - os_images_build | bool
+    - os_images_list |
+      selectattr("elements", "defined") |
+      selectattr("architecture", "defined") |
+      rejectattr("architecture", "equalto", ansible_facts.architecture) |
+      list | length > 0