add deployment strategy option to helm chart (#3021)

@S7evinK minor update to the helm chart on top of you existing fixes to
allow setting the update strategy as the default `RollingUpdate` one is
a bit annoying if using `ReadWriteOnce` volumes for media. Hope this
makes sense.

### 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~~ Haven't touched any go files.
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name](https://matrix-org.github.io/dendrite/development/contributing#sign-off)
_or_ I have already signed off privately

Signed-off-by: `George Antoniadis <george@noodles.gr>`
[skip ci]

helm/dendrite/templates/deployment.yaml

@@ -19,6 +19,13 @@ spec:
       annotations:
         confighash: secret-{{ .Values.dendrite_config | toYaml | sha256sum | trunc 32 }}
     spec:
+      strategy:
+        type: {{ $.Values.strategy.type }}
+        {{- if eq $.Values.strategy.type "RollingUpdate" }}
+        rollingUpdate:
+          maxSurge: {{ $.Values.strategy.rollingUpdate.maxSurge }}
+          maxUnavailable: {{ $.Values.strategy.rollingUpdate.maxUnavailable }}
+        {{- end }}
       volumes:
       - name: {{ include "dendrite.fullname" . }}-conf-vol
         secret:

helm/dendrite/values.yaml

@@ -43,6 +43,16 @@ persistence:
     # -- PVC Storage Request for the search volume
     capacity: "1Gi"
 
+strategy:
+  # -- Strategy to use for rolling updates (e.g. Recreate, RollingUpdate)
+  # If you are using ReadWriteOnce volumes, you should probably use Recreate
+  type: RollingUpdate
+  rollingUpdate:
+    # -- Maximum number of pods that can be unavailable during the update process
+    maxUnavailable: 25%
+    # -- Maximum number of pods that can be scheduled above the desired number of pods
+    maxSurge: 25%
+
 dendrite_config:
   version: 2
   global: