[azp]: Fix type in slave container cleanup (#10424)

Signed-off-by: Lawrence Lee <lawlee@microsoft.com>

Author: theasianpianist

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

@@ -1,7 +1,7 @@
 steps:
 - script: |
     containers=$(docker container ls -a | grep "sonic-slave" | awk '{ print $1 }')
-    [ -n "$containers" ] && docker container rm -f containers
+    [ -n "$containers" ] && docker container rm -f $containers
     docker images | grep "^<none>" | awk '{print$3}' | xargs -i docker rmi {}
     images=$(docker images 'sonic-slave-*' -a -q)
     [ -n "$images" ] && docker rmi -f $images