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

api/v1alpha1/runner_types.go

@@ -45,6 +45,9 @@ type RunnerSpec struct {
 	// +optional
 	Group string `json:"group,omitempty"`
 
+	// +optional
+	Ephemeral *bool `json:"ephemeral,omitempty"`
+
 	// +optional
 	Containers []corev1.Container `json:"containers,omitempty"`
 	// +optional

api/v1alpha1/zz_generated.deepcopy.go

@@ -587,6 +587,11 @@ func (in *RunnerSpec) DeepCopyInto(out *RunnerSpec) {
 		*out = make([]string, len(*in))
 		copy(*out, *in)
 	}
+	if in.Ephemeral != nil {
+		in, out := &in.Ephemeral, &out.Ephemeral
+		*out = new(bool)
+		**out = **in
+	}
 	if in.Containers != nil {
 		in, out := &in.Containers, &out.Containers
 		*out = make([]v1.Container, len(*in))