fix(helm): file permission issues when running as non-root user

Specify the user group for the garage (and init) process and ensure
that the persistent storage is mounted with the correct file system
group

script/helm/garage/templates/workload.yaml

@@ -41,6 +41,8 @@ spec:
                 secretKeyRef:
                   name: {{ include "garage.rpcSecretName" . }}
                   key: rpcSecret
+          securityContext:
+            {{- toYaml .Values.securityContext | nindent 12 }}
           volumeMounts:
             - name: configmap
               mountPath: /mnt/garage.toml