update docker.yml build policy (#183)

Author: cyjseagull

.github/workflows/docker.yml

@@ -7,9 +7,12 @@ on:
       - "README.md"
     tags:
       - 'v3.*.*'
-    branches:
-      - main
-      - feature-milestone2
+  pull_request:
+    paths-ignore:
+      - "docs/**"
+      - "python/**"
+      - "Changelog.md"
+      - "README.md"
   release:
     types: [prereleased]
 
@@ -25,6 +28,7 @@ env:
 
 jobs:
   build_docker:
+    if: github.event_name == 'pull_request' && github.event.action == 'closed' && github.event.pull_request.merged == true
     runs-on: ubuntu-latest
     steps:
       - name: Checkout