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>

unittests/init/init_directory_structure.sh-rootless.yaml

@@ -66,3 +66,23 @@ tests:
               chmod 700 "${GNUPGHOME}"
               chown 1000:1000 "${GNUPGHOME}"
             fi
+  - it: it does not chown /data even when image.fullOverride is set
+    template: templates/gitea/init.yaml
+    set:
+      image.fullOverride: gitea/gitea:1.20.5
+    asserts:
+      - equal:
+          path: stringData["init_directory_structure.sh"]
+          value: |-
+            #!/usr/bin/env bash
+
+            set -euo pipefail
+
+            set -x
+            mkdir -p /data/git/.ssh
+            chmod -R 700 /data/git/.ssh
+            [ ! -d /data/gitea/conf ] && mkdir -p /data/gitea/conf
+
+            # prepare temp directory structure
+            mkdir -p "${GITEA_TEMP}"
+            chmod ug+rwx "${GITEA_TEMP}"