Make build-all-images work on arm

contrib/DockerFileBuildHelper/StringBuilderEx.cs

@@ -0,0 +1,45 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+
+namespace DockerFileBuildHelper
+{
+    public class StringBuilderEx
+    {
+        StringBuilder _Builder = new StringBuilder();
+        public StringBuilderEx()
+        {
+
+        }
+
+        public int Indent { get; set; }
+
+        public void Append(string str)
+        {
+            _Builder.Append(GetIndents());
+            _Builder.Append(str);
+        }
+
+        private string GetIndents()
+        {
+            return new String(Enumerable.Range(0, Indent).Select(_ => '\t').ToArray());
+        }
+
+        public void AppendLine(string str)
+        {
+            _Builder.Append(GetIndents());
+            _Builder.AppendLine(str);
+        }
+
+        public override string ToString()
+        {
+            return _Builder.ToString();
+        }
+
+        internal void AppendLine()
+        {
+            _Builder.AppendLine();
+        }
+    }
+}