Update nginx Docker tag to v1.23.1

Reviewed-on: #1

Dockerfile

@@ -1,4 +1,4 @@
-FROM nginx:1.21.4
+FROM nginx:1.23.1
 # TODO: use renovate-bot to send pull requests on upstream image updates
 
 RUN mkdir /etc/nginx/templates.override

renovate.json

@@ -0,0 +1,3 @@
+{
+  "$schema": "https://docs.renovatebot.com/renovate-schema.json"
+}