Apply suggestions from code review

Author: gplanchat

src/Dockerfile/Arg.php

@@ -8,12 +8,15 @@
 {
     public function __construct(
         private string $name,
-        private string $defaultValue,
+        private ?string $defaultValue = null,
     ) {
     }
 
     public function __toString(): string
     {
-        return sprintf('ARG %s=%s', $this->name, $this->defaultValue);
+        if ($this->defaultValue !== null) {
+            return sprintf('ARG %s=%s', $this->name, $this->defaultValue);
+        }
+        return sprintf('ARG %s', $this->name);
     }
 }