Add image.fullOverride (#550)

### Description of the change

This PR is a continuation of the work done by @dgershman in [534](https://gitea.com/gitea/helm-chart/pulls/534), to allow users to override the image from the default rootless behavior of appending `-rootless` to the end of the image tag.

### Benefits

Allows more flexibility to use externally maintained images that are rootless but don't follow the `-rootless` tag convention.

### Applicable issues

- fixes #532

### Additional information

No breaking changes. This does not affect the `image.rootless` conditional checks or the current behavior if someone still wants to rely on the chart to append `-rootless`.

### 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)
- [x] Breaking changes are documented in the `README.md`
- [x] Templating unittests are added

Co-authored-by: TristanHoladay <40547442+TristanHoladay@users.noreply.github.com>
Co-authored-by: pat-s <pat-s@noreply.gitea.com>
Reviewed-on: https://gitea.com/gitea/helm-chart/pulls/550
Reviewed-by: justusbunsi <justusbunsi@noreply.gitea.com>
Reviewed-by: pat-s <pat-s@noreply.gitea.com>
Co-authored-by: TristanHoladay <tristanholaday@noreply.gitea.com>
Co-committed-by: TristanHoladay <tristanholaday@noreply.gitea.com>

templates/_helpers.tpl

@@ -56,6 +56,7 @@ Create chart name and version as used by the chart label.
 Create image name and tag used by the deployment.
 */}}
 {{- define "gitea.image" -}}
+{{- $fullOverride := .Values.image.fullOverride | default "" -}}
 {{- $registry := .Values.global.imageRegistry | default .Values.image.registry -}}
 {{- $repository := .Values.image.repository -}}
 {{- $separator := ":" -}}
@@ -65,7 +66,9 @@ Create image name and tag used by the deployment.
 {{- if .Values.image.digest }}
     {{- $digest = (printf "@%s" (.Values.image.digest | toString)) -}}
 {{- end -}}
-{{- if $registry }}
+{{- if $fullOverride }}
+    {{- printf "%s" $fullOverride -}}
+{{- else if $registry }}
     {{- printf "%s/%s%s%s%s%s" $registry $repository $separator $tag $rootless $digest -}}
 {{- else -}}
     {{- printf "%s%s%s%s%s" $repository $separator $tag $rootless $digest -}}