Remove all non-alphanumeric characters from the hostId. Fixes ##1006

src/WebJobs.Script.WebHost/App_Start/WebHostResolver.cs

@@ -6,6 +6,7 @@
 using System.IO;
 using System.Linq;
 using System.Reflection;
+using System.Text;
 using System.Threading.Tasks;
 using Microsoft.Azure.WebJobs.Script.Config;
 using Microsoft.Azure.WebJobs.Script.WebHost.WebHooks;
@@ -151,8 +152,17 @@ private static ScriptHostConfiguration CreateScriptHostConfiguration(WebHostSett
 
             // If running on Azure Web App, derive the host ID from the default subdomain
             // Otherwise, derive it from machine name and folder name
-            string hostId = _settingsManager.AzureWebsiteDefaultSubdomain
-                ?? $"{Environment.MachineName}-{Path.GetFileName(Environment.CurrentDirectory)}";
+            string hostId = _settingsManager.AzureWebsiteDefaultSubdomain;
+            if (string.IsNullOrEmpty(hostId))
+            {
+                var sanitizedPath = Path.GetFileName(Environment.CurrentDirectory)
+                    .Where(char.IsLetterOrDigit)
+                    .Aggregate(new StringBuilder(), (b, c) => b.Append(c))
+                    .ToString();
+
+                hostId = $"{Environment.MachineName}-{sanitizedPath}";
+            }
+
             if (!String.IsNullOrEmpty(hostId))
             {
                 // Truncate to the max host name length if needed