update the host files to point to accessible docker container names for backends

added testing for this, including mock backend server.

dockerfiles/services/sslproxy/hosts/xaibox.xai-corp.net.conf

@@ -16,6 +16,9 @@ server {
     ssl_certificate_key /etc/letsencrypt/live/xai-corp.net/privkey.pem;
 
     client_max_body_size 200m;
+
+    # this is the internal Docker DNS, cache only for 30s
+    resolver 127.0.0.11 valid=5s;
     set $backend http://xaibox_app;
 
     #Strict-Transport-Security: max-age=15768000