Add gpg configuration settings (#343)

### Description of the change

This PR adds support for gpg key setup. It allows to pass the gpg private key content inline inside `values.yaml` or refer to an existing secret containing the key content data.

### Benefits

Administrators don't need to manually setup the gpg environment from inside a running container. It also eliminates the breaking change of Gitea 1.17 regarding `[git].HOME` as the `GNUPGHOME` environment variable is used consistently to relocate the `.gnupg` directory to its former location.

### Applicable issues

  - fixes #107

### Additional information

This PR add the first unit tests to this Helm Chart, ensuring templating integrity for signing related configuration.

### 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)

Co-authored-by: justusbunsi <sk.bunsenbrenner@gmail.com>
Co-authored-by: pat-s <pat-s@noreply.gitea.io>
Reviewed-on: https://gitea.com/gitea/helm-chart/pulls/343
Reviewed-by: luhahn <luhahn@noreply.gitea.io>
Reviewed-by: techknowlogick <techknowlogick@gitea.io>
Co-authored-by: justusbunsi <justusbunsi@noreply.gitea.io>
Co-committed-by: justusbunsi <justusbunsi@noreply.gitea.io>

templates/gitea/init.yaml

@@ -6,6 +6,11 @@ metadata:
     {{- include "gitea.labels" . | nindent 4 }}
 type: Opaque
 stringData:
+  configure_gpg_environment.sh: |-
+    #!/usr/bin/env bash
+    set -eu
+
+    gpg --import /raw/private.asc
   init_directory_structure.sh: |-
     #!/usr/bin/env bash
 
@@ -35,6 +40,14 @@ stringData:
     {{- end }}
     chmod ug+rwx "${GITEA_TEMP}"
 
+    {{ if .Values.signing.enabled -}}
+    if [ ! -d "${GNUPGHOME}" ]; then
+      mkdir -p "${GNUPGHOME}"
+      chmod 700 "${GNUPGHOME}"
+      chown 1000:1000 "${GNUPGHOME}"
+    fi
+    {{- end }}
+
   configure_gitea.sh: |-
     #!/usr/bin/env bash