[docker-ptf] Modify docker container pull setting (#21560)

Set pull to 'missing' to avoid pulling and overwriting the local image if there is a preloaded image present. This is used for testing PR built image which is preloaded in elastic test when PTF_MODIFIED is True. This mechanism is used to test PR built docker-ptf image.

Signed-off-by: opcoder0 <[email protected]>

Author: opcoder0

ansible/roles/vm_set/tasks/add_topo.yml

@@ -167,7 +167,12 @@
     docker_container:
       name: ptf_{{ vm_set_name }}
       image: "{{ docker_registry_host }}/{{ ptf_imagename }}:{{ ptf_imagetag }}"
-      pull: yes
+      # Set pull to 'missing' when ptf_modified is True (PR test with preloaded image)
+      # to avoid pulling and overwriting the local image. For nightly tests where
+      # ptf_modified is False/undefined, use 'always' to always pull the latest image.
+      # The ptf_modified flag is passed from Azure Pipelines through Elastictest's
+      # add_topo_params as "-e ptf_modified=True" when testing PR built docker-ptf images.
+      pull: "{{ 'missing' if (ptf_modified | default(false) | bool) else 'always' }}"
       state: started
       restart: no
       network_mode: none