[SPARK-27626][K8S] Fix docker-image-tool.sh to be robust in non-bash shell env

Author: dongjoon-hyun

bin/docker-image-tool.sh

@@ -282,7 +282,7 @@ do
    if ! minikube status 1>/dev/null; then
      error "Cannot contact minikube. Make sure it's running."
    fi
-   eval $(minikube docker-env)
+   eval $(minikube docker-env --shell bash)
    ;;
   u) SPARK_UID=${OPTARG};;
  esac