Helm chart - Add configuration for image pull secrets (#3428)

This minor change should allow the use of private container images with
this chart by specifying image pull secrets.

### 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>

Author: Arkaniad

Diff for: helm/dendrite/Chart.yaml

@@ -1,6 +1,6 @@
 apiVersion: v2
 name: dendrite
-version: "0.14.2"
+version: "0.14.3"
 appVersion: "0.13.8"
 description: Dendrite Matrix Homeserver
 type: application

Diff for: helm/dendrite/templates/deployment.yaml

@@ -110,3 +110,7 @@ spec:
           httpGet:
             path: /_dendrite/monitor/up
             port: http
+      imagePullSecrets:
+      {{- with .Values.imagePullSecrets }}
+      {{ . | toYaml | nindent 6 }}
+      {{- end }}

Diff for: helm/dendrite/values.yaml

@@ -6,6 +6,10 @@ image:
   # -- Overrides the image tag whose default is the chart appVersion.
   tag: ""
 
+# -- Configure image pull secrets to use private container registry
+# https://kubernetes.io/docs/tasks/configure-pod-container/pull-image-private-registry/#create-a-pod-that-uses-your-secret
+imagePullSecrets: []
+# - name: your-pull-secret-name
 
 # signing key to use
 signing_key: