Add securitycontext settings on defaultbackend

Signed-off-by: Alexis Martinier <a.martinier@gmail.com>

charts/ingress-nginx/values.yaml

@@ -552,6 +552,9 @@ defaultBackend:
     pullPolicy: IfNotPresent
     # nobody user -> uid 65534
     runAsUser: 65534
+    runAsNonRoot: true
+    readOnlyRootFilesystem: true
+    allowPrivilegeEscalation: false
 
   extraArgs: {}