try with Error Stream redirection

.appveyor.yml

@@ -61,7 +61,7 @@ before_test:
       $Counter=0
       $Found=$false
       While ( ! $Found -and $Counter -lt 60 ) {
-        $Found = ( docker logs openldap | Select-String -Quiet "\*\* Starting slapd \*\*" )
+        $Found = ( docker logs openldap 2>&1 | Select-String -Quiet "\*\* Starting slapd \*\*" )
         Start-Sleep -Seconds 1
         $Counter++
         "$Counter Waiting for slapd"