fix: Initialize _containerId with None to prevent NullPointerException

Fixed uninitialized var _containerId which was causing NullPointerException
in Scala 3 environment. The variable is now properly initialized with None.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: j5ik2o

docker-controller-scala-core/src/main/scala/com/github/j5ik2o/dockerController/DockerController.scala

@@ -90,7 +90,7 @@ private[dockerController] class DockerControllerImpl(
 
   protected val logger: Logger = LoggerFactory.getLogger(getClass)
 
-  private var _containerId: Option[String] = _
+  private var _containerId: Option[String] = None
 
   private def repoTag: String = tag.fold(imageName)(t => s"$imageName:$t")