[ci] Fix issues in docker-slave official build. (#10477)

only push latest tag on master build

.azure-pipelines/docker-sonic-slave-template.yml

@@ -101,10 +101,13 @@ jobs:
       containerRegistry: ${{ parameters.registry_conn }}
       repository: $(VARIABLE_SLAVE_BASE_IMAGE)
       command: push
-      tags: |
-        $(VARIABLE_SLAVE_BASE_TAG)
-        ${{ if eq(variables['Build.SourceBranchName'], 'master') }}:
+      ${{ if eq(variables['Build.SourceBranchName'], 'master') }}:
+        tags: |
+          $(VARIABLE_SLAVE_BASE_TAG)
           latest
+      ${{ else }}:
+        tags: |
+          $(VARIABLE_SLAVE_BASE_TAG)
   - ${{ if ne(parameters.arch, 'amd64') }}:
     - task: Docker@2
       condition: ne(variables['Build.Reason'], 'PullRequest')

.azure-pipelines/docker-sonic-slave.yml

@@ -60,7 +60,7 @@ stages:
   - ${{ each dist in parameters.dists }}:
     - ${{ if endswith(variables['Build.DefinitionName'], dist) }}:
       - ${{ each arch in parameters.arches }}:
-        - ${{ if eq(variables['System.PullRequest.TargetBranch'], 'master') }}:
+        - ${{ if eq(variables[''Build.Reason], 'PullRequest') }}:
           - template: docker-sonic-slave-template.yml
             parameters:
               pool: sonicbld