Made from image name in From optional, Issue #179

Ductus.FluentDocker/Builders/ImageBuilder.cs

@@ -61,8 +61,23 @@ protected override IBuilder InternalCreate()
       return new ImageBuilder(this);
     }
 
-    public FileBuilder From(string from)
-    {
+    /// <summary>
+    /// Creates a _dockerfile_ builder.
+    /// </summary>
+    /// <param name="from">
+    /// Optional image to specify as FROM. If omitted, it is up to the caller to specify _UseParent_ or _From_.
+    /// </param>
+    /// <returns>
+    /// A newly created filebuilder. If empty or null string the `FileBuilder` is empty. Otherwise it has populated
+    /// the `FileBuilder` with a parent of the specified image name (via _UseParent()_).
+    /// </returns>
+    public FileBuilder From(string from = null)
+    {
+      if (string.IsNullOrEmpty(from))
+      {
+        return _fileBuilder = new FileBuilder(this);
+      }
+
       return _fileBuilder = new FileBuilder(this).UseParent(from);
     }