nagios: Add a Nagios image for service monitoring

The lighttpd config snippet is based on Gentoo's
lighttpd_nagios3-r1.conf from net-analyzer/nagios-core.  I've dropped
the auth snippets (you can put your auth in a reverse-proxy in front
of the nagios container) and added a redirect from / to /nagios.

I had to add mod_fastcgi.conf to avoid:

  NOT handling file as static file, extension forbidden

for *.php files.

I set timezones to avoid the following PHP warning:

  date(): It is not safe to rely on the system's timezone
  settings. You are *required* to use the date.timezone setting or the
  date_default_timezone_set() function. In case you used any of those
  methods and you are still getting this warning, you most likely
  misspelled the timezone identifier