ci: T9047: dispatch docker-image rebuild against production ref

The Docker image rebuild curl-dispatch was still pointed at
vyos-reusable-workflows main, which is frozen pre-App-token-migration and
still uses the retired secrets.PAT checkout pattern (empty token, immediate
"could not read Username" auth failure on every dispatched run since
2026-06-02). The fix already shipped and is live on production
(VyOS-Networks/vyos-reusable-workflows#123, merged 2026-05-31) — this
caller was simply never repointed.

🤖 Generated by [robots](https://vyos.io)

Author: andamasov

.github/workflows/trigger-docker-image-build.yml

@@ -22,7 +22,7 @@ jobs:
     runs-on: ubuntu-latest
 
     env:
-      REF: main  # Used for curl to trigger image build
+      REF: production  # Used for curl to trigger image build
 
     steps:
       - name: Checkout vyos/vyos-build repo