Faster refresh of DNS.

The Docker resolver replies with a TTL of 600, too long. Refresh after
30 seconds.

conf.d/global.conf

 # The resolver for the Docker network.
-resolver        127.0.0.11;
+resolver        127.0.0.11 valid=30s;
 gzip            on;
 tcp_nopush      on;
 tcp_nodelay     on;