### 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>
