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>

templates/gitea/deployment.yaml

@@ -340,6 +340,9 @@ spec:
               subPath: {{ .Values.persistence.subPath }}
               {{- end }}
             {{- include "gitea.container-additional-mounts" . | nindent 12 }}
+        {{- if .Values.extraContainers }}
+        {{- toYaml .Values.extraContainers | nindent 8 }}
+        {{- end }}
       {{- with .Values.global.hostAliases }}
       hostAliases:
         {{- toYaml . | nindent 8 }}
@@ -403,4 +406,4 @@ spec:
   {{- else if not .Values.persistence.enabled }}
         - name: data
           emptyDir: {}
-  {{- end }}
+  {{- end }}