feat(act_runner): allow to mount volumes (#756)

### Description of the change

In the act_runner StatefulSet, this allows to mount volumes.

### Benefits

It allows to mount some volumes in any of the two containers, e.g. certificates for a private registry.

### Possible drawbacks

I can't think of any.

### Applicable issues

- Fixes #744

### Checklist

- [X] Parameters are documented in the `values.yaml` and added to the `README.md` using [readme-generator-for-helm](https://github.com/bitnami-labs/readme-generator-for-helm)
- [X] Helm templating unittests are added (required when changing anything in `templates` folder)

Co-authored-by: Xav <grandmou@protonmail.com>
Reviewed-on: https://gitea.com/gitea/helm-chart/pulls/756
Reviewed-by: justusbunsi <justusbunsi@noreply.gitea.com>
Co-authored-by: grandmou <grandmou@noreply.gitea.com>
Co-committed-by: grandmou <grandmou@noreply.gitea.com>

unittests/helm/act_runner/statefulset.yaml

@@ -128,3 +128,55 @@ tests:
           value:
             name: "CUSTOM_ENV_NAME"
             value: "custom env value"
+  - it: should mount an extra volume in the act runner container
+    template: templates/gitea/act_runner/statefulset.yaml
+    set:
+      actions:
+        enabled: true
+        statefulset:
+          extraVolumes:
+            - name: my-act-runner-volume
+              emptyDir: {}
+          actRunner:
+            extraVolumeMounts:
+              - mountPath: /mnt
+                name: my-act-runner-volume
+    asserts:
+      - hasDocuments:
+          count: 1
+      - containsDocument:
+          kind: StatefulSet
+          apiVersion: apps/v1
+          name: gitea-unittests-act-runner
+      - contains:
+          any: true
+          path: spec.template.spec.containers[0].volumeMounts
+          content:
+            mountPath: /mnt
+            name: my-act-runner-volume
+  - it: should mount an extra volume in the docker-in-docker container
+    template: templates/gitea/act_runner/statefulset.yaml
+    set:
+      actions:
+        enabled: true
+        statefulset:
+          extraVolumes:
+            - name: my-dind-volume
+              emptyDir: {}
+          dind:
+            extraVolumeMounts:
+              - mountPath: /mnt
+                name: my-dind-volume
+    asserts:
+      - hasDocuments:
+          count: 1
+      - containsDocument:
+          kind: StatefulSet
+          apiVersion: apps/v1
+          name: gitea-unittests-act-runner
+      - contains:
+          any: true
+          path: spec.template.spec.containers[1].volumeMounts
+          content:
+            mountPath: /mnt
+            name: my-dind-volume