Pass IMAGE_TAG to omnibus-gitlab-mirror trigger

Signed-off-by: Balasankar "Balu" C <balasankarc@autistici.org>

scripts/trigger-build

@@ -138,8 +138,12 @@ module Trigger
     def extra_variables
       # Use CI_MERGE_REQUEST_SOURCE_BRANCH_SHA for omnibus checkouts due to pipeline for merged results
       # and fallback to CI_COMMIT_SHA for the `detached` pipelines.
+      # We also set IMAGE_TAG so the GitLab and QA docker images are tagged with
+      # that SHA.
+      source_sha = Trigger.non_empty_variable_value('CI_MERGE_REQUEST_SOURCE_BRANCH_SHA') || ENV['CI_COMMIT_SHA']
       {
-        'GITLAB_VERSION' => Trigger.non_empty_variable_value('CI_MERGE_REQUEST_SOURCE_BRANCH_SHA') || ENV['CI_COMMIT_SHA'],
+        'GITLAB_VERSION' => source_sha,
+        'IMAGE_TAG' => source_sha,
         'ALTERNATIVE_SOURCES' => 'true',
         'SECURITY_SOURCES' => Trigger.security? ? 'true' : 'false',
         'ee' => Trigger.ee? ? 'true' : 'false',