Helm chart - Fix image pull secrets for signing-key job (#3430)

Forgotten in https://github.com/matrix-org/dendrite/pull/3428 - Pull
secrets need to be used for signing-key job since it uses Dendrite
container as well

### Pull Request Checklist

<!-- Please read
https://matrix-org.github.io/dendrite/development/contributing before
submitting your pull request -->

* [x] I have added Go unit tests or [Complement integration
tests](https://github.com/matrix-org/complement) for this PR _or_ I have
justified why this PR doesn't need tests
* [x] Pull request includes a [sign off below using a legally
identifiable
name](https://matrix-org.github.io/dendrite/development/contributing#sign-off)
_or_ I have already signed off privately

Signed-off-by: `Rhea Danzey <rdanzey@element.io>`

---------

Signed-off-by: Rhea Danzey <rdanzey@element.io>

helm/dendrite/Chart.yaml

@@ -1,6 +1,6 @@
 apiVersion: v2
 name: dendrite
-version: "0.14.4"
+version: "0.14.5"
 appVersion: "0.13.8"
 description: Dendrite Matrix Homeserver
 type: application

helm/dendrite/templates/jobs.yaml

@@ -54,6 +54,10 @@ metadata:
 spec:
   template:
     spec:
+      imagePullSecrets:
+      {{- with .Values.imagePullSecrets }}
+      {{ . | toYaml | nindent 6 }}
+      {{- end }}
       restartPolicy: "Never"
       serviceAccount: {{ $name }}
       containers: