chore: update workflow only for tags

Author: Secbone

.github/workflows/package.yaml

@@ -1,6 +1,9 @@
 name: build image to gihub packages
 
-on: push
+on:
+  push:
+    tags:
+    - "*"
 
 env:
   REGISTRY: ghcr.io

README.md

@@ -25,9 +25,9 @@ MacOS 打开 `日历`，左上角 `文件` - `新建日历订阅`，添加如下
 
 自建订阅服务
 ```bash
-docker pull ghcr.io/secbone/calendar:master
+docker pull ghcr.io/secbone/calendar
 
-docker run -d -p [port]:80 ghcr.io/secbone/calendar:master
+docker run -d -p [port]:80 ghcr.io/secbone/calendar
 ```
 
 ## Thanks