Add support for image.digest (#444)

fix #398

Co-authored-by: justusbunsi <sk.bunsenbrenner@gmail.com>
Reviewed-on: https://gitea.com/gitea/helm-chart/pulls/444
Reviewed-by: justusbunsi <justusbunsi@noreply.gitea.com>
Co-authored-by: pat-s <patrick.schratz@gmail.com>
Co-committed-by: pat-s <patrick.schratz@gmail.com>

values.yaml

@@ -40,6 +40,7 @@ clusterDomain: cluster.local
 ## @param image.registry image registry, e.g. gcr.io,docker.io
 ## @param image.repository Image to start for this pod
 ## @param image.tag Visit: [Image tag](https://hub.docker.com/r/gitea/gitea/tags?page=1&ordering=last_updated). Defaults to `appVersion` within Chart.yaml.
+## @param image.digest Image digest. Allows to pin the given image tag. Useful for having control over mutable tags like `latest`
 ## @param image.pullPolicy Image pull policy
 ## @param image.rootless Wether or not to pull the rootless version of Gitea, only works on Gitea 1.14.x or higher
 image:
@@ -47,6 +48,7 @@ image:
   repository: gitea/gitea
   # Overrides the image tag whose default is the chart appVersion.
   tag: ""
+  digest: ""
   pullPolicy: Always
   rootless: true