feat: allow ephemeral runner to be optional (#498)

- Adds `ephemeral` option to `runner.spec` 
    
    ```
      ....
      template:
         spec:
             ephemeral: false
             repository: mumoshu/actions-runner-controller-ci
      ....
    ```
- `ephemeral` defaults to `true`
- `entrypoint.sh` in runner/Dockerfile modified to read `RUNNER_EPHEMERAL` flag
- Runner images are backward-compatible. `--once` is omitted only when the new envvar `RUNNER_EPHEMERAL` is explicitly set to `false`.

Resolves #457

controllers/runner_controller.go

@@ -567,6 +567,7 @@ func (r *RunnerReconciler) newPod(runner v1alpha1.Runner) (corev1.Pod, error) {
 		privileged      bool = true
 		dockerdInRunner bool = runner.Spec.DockerdWithinRunnerContainer != nil && *runner.Spec.DockerdWithinRunnerContainer
 		dockerEnabled   bool = runner.Spec.DockerEnabled == nil || *runner.Spec.DockerEnabled
+		ephemeral       bool = runner.Spec.Ephemeral == nil || *runner.Spec.Ephemeral
 	)
 
 	runnerImage := runner.Spec.Image
@@ -625,6 +626,10 @@ func (r *RunnerReconciler) newPod(runner v1alpha1.Runner) (corev1.Pod, error) {
 			Name:  "RUNNER_WORKDIR",
 			Value: workDir,
 		},
+		{
+			Name:  "RUNNER_EPHEMERAL",
+			Value: fmt.Sprintf("%v", ephemeral),
+		},
 	}
 
 	if metav1.HasAnnotation(runner.ObjectMeta, annotationKeyRegistrationOnly) {