Add $(IMAGE) to the release-manifests target. This will allow e2e testing of a release prior to cutting the actual release. For example, @Xunzhuo created this test v0.2.0 release but the EG deployment manifest uses a non-existent image.

$ kubectl get deploy/envoy-gateway -n envoy-gateway-system -o yaml | grep "image: "
        image: envoyproxy/gateway:v0.2.0