add network exists check

Author: ntrappe-msft

helpful_tools/Install-DockerCE/install-docker-ce.ps1

@@ -250,6 +250,14 @@ Install-Feature
 function
 New-ContainerTransparentNetwork
 {
+    # Check if transparent network already created
+    $networkList = docker network ls 
+    if ($networkList -match '\bTransparent\b') {
+        Write-Output "Network with the name Transparent exists."
+        return
+    }
+ 
+    # Continue with network creation
     if ($ExternalNetAdapter)
     {
         $netAdapter = (Get-NetAdapter |? {$_.Name -eq "$ExternalNetAdapter"})[0]