Add capability to filter which NTP-clients are allowed to query this chrony NTP-server

[Apachez-]

As spinoff to #43.

Would be handy to define through an environment variable a list of NTP-clients who are allowed to query this specific chrony NTP-server.

Something like this:

• NTP_CLIENTS_ALLOWED: default empty (allow all). Add prefixes that shall be allowed to query chrony. When defined any prefixes not part of this allowed-list will be denied.

Example:

set container name ntp environment NTP_CLIENTS_ALLOWED value '10.0.0.0/8,172.16.0.0/12,192.168.0.0/16'

Ref:

https://chrony-project.org/doc/latest/chrony.conf.html