Add 'extraContainers' parameter (#697)

### Description of the change

Adds an 'extraContainers' parameter.

### Benefits

Users will be able to run sidecar containers as required by their environment.

### Possible drawbacks

N/A

### Applicable issues

- Fixes #696

### 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)

Reviewed-on: https://gitea.com/gitea/helm-chart/pulls/697
Reviewed-by: justusbunsi <justusbunsi@noreply.gitea.com>
Co-authored-by: rossigee <rossigee@noreply.gitea.com>
Co-committed-by: rossigee <rossigee@noreply.gitea.com>

unittests/deployment/sidecar-container.yaml

@@ -0,0 +1,21 @@
+suite: sidecar container
+release:
+  name: gitea-unittests
+  namespace: testing
+templates:
+  - templates/gitea/deployment.yaml
+  - templates/gitea/config.yaml
+tests:
+  - it: supports adding a sidecar container
+    template: templates/gitea/deployment.yaml
+    set:
+      extraContainers:
+        - name: sidecar-bob
+          image: busybox
+    asserts:
+      - equal:
+          path: spec.template.spec.containers[1].name
+          value: "sidecar-bob"
+      - equal:
+          path: spec.template.spec.containers[1].image
+          value: "busybox"