Add CNAME option

When a repo have a CNAME file, it will use the URL on it to map the server_name on ngnix

plugins/nginx-vhosts/post-deploy

@@ -25,6 +25,10 @@ EOF
     SSL_DIRECTIVES=""
   fi
 
+  if [[ -f "$DOKKU_ROOT/$APP/CNAME" ]]; then
+    hostname=$(< "$DOKKU_ROOT/$APP/CNAME" )
+  fi
+
   # ssl based nginx.conf
   if [[ -n "$SSL_INUSE" ]]; then
   cat<<EOF > $DOKKU_ROOT/$APP/nginx.conf