./sbin/update-dns.sh may be called at regular intervals (one hour
or so) via crontab.
It will automatically detect:
- its local host name (*.lan.dougal.aaltronav.eu); and
- which IP has internet access, if any.
Armed with that information and with the dynamic DNS API password
stored in DYNDNS_PASSWD in ~/.dougalrc, it will update the relevant
DNS record.
For this to work, the first `lan.dougal` hostname in the Nginx
configuration must be the one that is set up for dynamic update.
Other `lan.dougal` hostnames should be CNAME records pointing to
the first one.
