GCP app engine flexible with custom runtime environment not working

Hi Guys,
I am trying to provision GCP AppEngine flexible with custom runtime where I will use my Dockerfile to install the runtime,

I am getting below error everytime whenever I am trying the create app engine

google_app_engine_flexible_app_version.myapp_flex: Creating...
google_app_engine_flexible_app_version.myapp_flex: Still creating... [10s elapsed]
╷
│ Error: Error waiting to create FlexibleAppVersion: Error waiting for Creating FlexibleAppVersion: Error code 3, message: "runtime: custom" requires either cloudbuild.yaml or Dockerfile to be present.

Following the official documentation of Harshicorp terraform its not mentioned about Dockerfile in which block we should define the docker file. I just keep my Dockerfile already in / dir.

https://registry.terraform.io/providers/hashicorp/google/latest/docs/resources/app_engine_flexible_app_version

resource "google_storage_bucket" "source-code" {

  name     = "source-code-gcs"    #bucket-name

  location = "US"

  force_destroy = true

}

data "archive_file" "app-engine-source-zip" {

 type        = "zip"

 source_dir  = "./source/app-engine/full-stack-app"      #source-code path

 output_path = "./full-stack-app.zip"

}

resource "google_storage_bucket_object" "app-engine-source-zip-obj" {

 name   = "source-app-engine.${data.archive_file.app-engine-source-zip.output_md5}.zip"

 bucket = google_storage_bucket.source-code.name

 source = data.archive_file.app-engine-source-zip.output_path

}

resource "google_app_engine_flexible_app_version" "myapp_flex" {

  version_id = "v1"

  project    =  var.project_id

  service    = "flex"

  runtime    = "custom"

  entrypoint {

    shell = "./build_and_run.sh"

  }

  deployment {

    zip {

      source_url = "https://storage.googleapis.com/${google_storage_bucket.source-code.name}/${google_storage_bucket_object.app-engine-source-zip-obj.name}"

    }

    cloud_build_options {

      app_yaml_path = "app.yaml"

    }

   }

  }

  liveness_check {

    path = "/"

  }

  readiness_check {

    path = "/"

  }

  env_variables = {

    port = "8081"

  }

  automatic_scaling {

    cool_down_period = "120s"

    cpu_utilization {

      target_utilization = 0.5

    }

  }

  delete_service_on_destroy = true

}