[action] cosign by container image name (not by tag)

- Signing by image name will only sign by image digest, according
to the documentation. https://docs.sigstore.dev/cosign/sign#sign-a-container-multiple-times

Makefile

@@ -203,14 +203,14 @@ define DOCKER_PUSH
 	# $(1) -- App Name
 	# $(2) -- Image Tag
 	docker push smallstep/$(1):$(2)
-	cosign sign -r smallstep/$(1):$(2)
 endef
 
 docker-tag:
 	$(call DOCKER_TAG,step-sds,$(VERSION))
 
 docker-push-tag: docker-tag
 	$(call DOCKER_PUSH,step-sds,$(VERSION))
+	cosign sign -r smallstep/step-sds
 
 docker-push-tag-latest:
 	$(call DOCKER_PUSH,step-sds,latest)