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/deployment/ssh-configuration.yaml

@@ -27,6 +27,18 @@ tests:
           content:
             name: SSH_LOG_LEVEL
             value: "DEBUG"
+  - it: supports overriding SSH log level (even when image.fullOverride set)
+    template: templates/gitea/deployment.yaml
+    set:
+      image.fullOverride: gitea/gitea:1.19.3
+      image.rootless: false
+      gitea.ssh.logLevel: "DEBUG"
+    asserts:
+      - contains:
+          path: spec.template.spec.containers[0].env
+          content:
+            name: SSH_LOG_LEVEL
+            value: "DEBUG"
   - it: skips SSH_LOG_LEVEL for rootless image
     template: templates/gitea/deployment.yaml
     set:
@@ -38,3 +50,15 @@ tests:
           any: true
           content:
             name: SSH_LOG_LEVEL
+  - it: skips SSH_LOG_LEVEL for rootless image (even when image.fullOverride set)
+    template: templates/gitea/deployment.yaml
+    set:
+      image.fullOverride: gitea/gitea:1.19.3
+      image.rootless: true
+      gitea.ssh.logLevel: "DEBUG" # explicitly defining a non-standard level here
+    asserts:
+      - notContains:
+          path: spec.template.spec.containers[0].env
+          any: true
+          content:
+            name: SSH_LOG_LEVEL