matrix-synapse: Add a default 120s startup probe

This should avoid initial setup failing if the cluster is running on
low-powered hardware or using slow storage for the database

charts/matrix-synapse/templates/deployment.yaml

@@ -130,6 +130,10 @@ spec:
           readinessProbe:
             {{- . | toYaml | nindent 12 }}
           {{- end }}
+          {{- with .Values.synapse.startupProbe }}
+          startupProbe:
+            {{- . | toYaml | nindent 12 }}
+          {{- end }}
           volumeMounts:
             - name: config
               mountPath: /synapse/config

charts/matrix-synapse/values.yaml

@@ -272,6 +272,14 @@ synapse:
       path: /health
       port: http
 
+  ## Startup probe configuration to use
+  ##
+  startupProbe:
+    failureThreshold: 12
+    httpGet:
+      path: /health
+      port: http
+
   ## Node selectors to set for the main Synapse pod.
   ##
   nodeSelector: {}