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/statefulset.yaml

@@ -59,6 +59,10 @@ spec:
             {{- if .Values.statefulset.env }}
             {{- toYaml .Values.statefulset.env | nindent 12 }}
             {{- end }}
+            {{- if .Values.signing.enabled }}
+            - name: GNUPGHOME
+              value: {{ .Values.signing.gpgHome }}
+            {{- end }}
           volumeMounts:
             - name: init
               mountPath: /usr/sbin
@@ -110,6 +114,36 @@ spec:
             {{- include "gitea.init-additional-mounts" . | nindent 12 }}
           securityContext:
             {{- toYaml .Values.containerSecurityContext | nindent 12 }}
+        {{- if .Values.signing.enabled }}
+        - name: configure-gpg
+          image: "{{ include "gitea.image" . }}"
+          command: ["/usr/sbin/configure_gpg_environment.sh"]
+          imagePullPolicy: {{ .Values.image.pullPolicy }}
+          securityContext:
+            {{- /* By default this container runs as user 1000 unless otherwise stated */ -}}
+            {{- $csc := deepCopy .Values.containerSecurityContext -}}
+            {{- if not (hasKey $csc "runAsUser") -}}
+            {{- $_ := set $csc "runAsUser" 1000 -}}
+            {{- end -}}
+            {{- toYaml $csc | nindent 12 }}
+          env:
+            - name: GNUPGHOME
+              value: {{ .Values.signing.gpgHome }}
+          volumeMounts:
+            - name: init
+              mountPath: /usr/sbin
+            - name: data
+              mountPath: /data
+              {{- if .Values.persistence.subPath }}
+              subPath: {{ .Values.persistence.subPath }}
+              {{- end }}
+            - name: gpg-private-key
+              mountPath: /raw
+              readOnly: true
+            {{- if .Values.extraVolumeMounts }}
+            {{- toYaml .Values.extraVolumeMounts | nindent 12 }}
+            {{- end }}
+        {{- end }}
         - name: configure-gitea
           image: "{{ include "gitea.image" . }}"
           command: ["/usr/sbin/configure_gitea.sh"]
@@ -305,6 +339,15 @@ spec:
         {{- end }}
         - name: temp
           emptyDir: {}
+        {{- if .Values.signing.enabled }}
+        - name: gpg-private-key
+          secret:
+            secretName: {{ include "gitea.gpg-key-secret-name" . }}
+            items:
+              - key: privateKey
+                path: private.asc
+            defaultMode: 0100
+        {{- end }}
   {{- if and .Values.persistence.enabled .Values.persistence.existingClaim }}
         - name: data
           persistentVolumeClaim: