Update nginx docker image to 1.17 (replicates core PR #4800) (#574)

diff --git a/helm/openwhisk/values.yaml b/helm/openwhisk/values.yaml
index af6ccf2..c155d26 100644
--- a/helm/openwhisk/values.yaml
+++ b/helm/openwhisk/values.yaml
@@ -226,7 +226,7 @@
 # Nginx configurations
 nginx:
   imageName: "nginx"
-  imageTag: "1.15"
+  imageTag: "1.17"
   imagePullPolicy: "IfNotPresent"
   replicaCount: 1
   restartPolicy: "Always"