chore(k8s): take the image reference from build-tools (#241)

## Summary

`k8s/deploy.yaml` takes the image from build-tools: `image: ${IMAGE}` instead of the hard-coded `oci.unbound.se/unboundsoftware/unbound-site:${COMMIT}`.

`deploy` substitutes `${IMAGE}` with `<registry>/<owner>/<name>:<tag>`, where the registry comes from the build-tools config and the tag is the value `${COMMIT}` gets today. The rendered reference is unchanged. Moving off `oci.unbound.se` later becomes one change in infra.

Validated on shiny/time-service#864: merged and deployed to AWS staging and Frostmoln staging with the same image. Prerequisites, all merged:
- unboundsoftware/infra#1808: registry in `BUILDTOOLS_FROSTMOLN_STAGING`
- shiny/acctest#952: acctest pins build-tools to the Gitea registry

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Reviewed-on: https://gitea.unbound.se/unboundsoftware/unbound-site/pulls/241

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: