Make Container techn optional

This change makes the `techn` argument to `Container` and friends optional, similar to how it is optional in `Rel`.

I am not very familiar with this project or plantuml in general, so please review carefully.

C4_Container.puml

@@ -53,18 +53,18 @@ endlegend
 ' ##################################
 
 !function $getContainer($label, $techn, $descr, $sprite)
-!if ($descr == "") && ($sprite == "")
-!return '=='+$label+'\n//<size:'+$TECHN_FONT_SIZE+'>['+$techn+']</size>//'
-!endif
-!if ($descr == "") && ($sprite != "")
-!return $getSprite($sprite)+'\n=='+$label+'\n//<size:'+$TECHN_FONT_SIZE+'>['+$techn+']</size>//'
-!endif
-!if ($descr != "") && ($sprite == "")
-!return '=='+$label+'\n//<size:'+$TECHN_FONT_SIZE+'>['+$techn+']</size>//\n\n '+$descr
-!endif
-!if ($descr != "") && ($sprite != "")
-!return $getSprite($sprite)+'\n=='+$label+'\n//<size:'+$TECHN_FONT_SIZE+'>['+$techn+']</size>//\n\n '+$descr
-!endif
+  !$container = ""
+  !if ($sprite != "")
+    !$container = $container + $getSprite($sprite)+'\n'
+  !endif
+  !$container = $container + '=='+$label
+  !if ($techn != "")
+    !$container = $container + '\n//<size:'+$TECHN_FONT_SIZE+'>['+$techn+']</size>//'
+  !endif
+  !if ($descr != "")
+    !$container = $container + '\n\n '+$descr
+  !endif
+  !return $container
 !endfunction
 
 !unquoted procedure Container($alias, $label, $techn="", $descr="", $sprite="", $tags="", $link="")