fix(www): don't get stuck on outdated nginx releases

nginx drops support for even release numbers every April, declaring
the next even release number stable:
https://www.nginx.com/blog/nginx-1-16-1-17-released/

This commit ensures that we always use the stable version.

www/Dockerfile

@@ -1,4 +1,4 @@
-FROM nginx:1.15-alpine
+FROM nginx:stable-alpine
 
 # Add dependencies for our scripts
 RUN apk add --no-cache bash openssl inotify-tools