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>

values.yaml

@@ -283,6 +283,12 @@ persistence:
   annotations:
     helm.sh/resource-policy: keep
 
+## @param extraContainers Additional sidecar containers to run in the pod
+extraContainers: []
+#  - name: sidecar-bob
+#    image: busybox
+#    command: [/bin/sh, -c, 'echo "Hello world"; sleep 86400']
+
 ## @param extraVolumes Additional volumes to mount to the Gitea deployment
 extraVolumes: []
 # - name: postgres-ssl-vol