fix

azurepipeline.yml

@@ -5,12 +5,20 @@ trigger:
 pr:
   branches:
     include:
-    - "*"
+
+variables:
+  - name: BUILD_BRANCH
+    ${{ if eq(variables['Build.Reason'], 'PullRequest') }}:
+      value: $(System.PullRequest.TargetBranch)
+    ${{ else }}:
+      value: $(Build.SourceBranchName)   - "*"
 
 jobs:
 - job: Build
   pool:
     vmImage: ubuntu-20.04
+  container:
+    image: sonicdev-microsoft.azurecr.io:443/sonic-slave-buster:$(BUILD_BRANCH)
 
   variables:
     DIFF_COVER_CHECK_THRESHOLD: 80