chore(k8s): take the image reference from build-tools

deploy substitutes ${IMAGE} with <registry>/<owner>/<name>:<tag> from the build-tools config, so the registry is set in one place (infra) instead of in every manifest. Renders the same reference as before. Same change as shiny/time-service#864.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

k8s/deploy.yaml

@@ -44,7 +44,7 @@ spec:
           periodSeconds: 5
           timeoutSeconds: 5
         imagePullPolicy: Always
-        image: oci.unbound.se/unboundsoftware/unbound-site:${COMMIT}
+        image: ${IMAGE}
         ports:
         - containerPort: 80
         resources: