Added secret_key env var to kube rc

kubernetes/zulip-rc.yml

@@ -98,6 +98,8 @@ spec:
           value: "example.com"
         - name: ZULIP_USER_PASS
           value: "123456789"
+        - name: SECRETS_secret_key
+          value: "REPLCAE_WITH_SECURE_SECRET_KEY"
         ports:
         - containerPort: 80
           name: http