ci: serialize PR image validate builds on self-hosted runners

Co-authored-by: cynarAI <cynarAI@users.noreply.github.com>

.github/workflows/publish-server-image.yml

@@ -57,6 +57,9 @@ jobs:
       contents: read
     strategy:
       fail-fast: false
+      # One build at a time: parallel matrix jobs on the same self-hosted runner
+      # contend for a single buildkitd and lose the daemon mid-build.
+      max-parallel: 1
       matrix:
         include:
           - name: app