infra: run local pkg caching server

[skhal]

Similar to local_unbound, we'd like to run a local caching PKG server. Jails should install software through this server to save traffic and speed up installations.

See poudriere server configuration:

lab/infra/net.lab.pkg/usr/local/etc/nginx/nginx.conf

Lines 40 to 53 in 00bca5b

	server {
	listen 10.0.0.6:80;
	server_name localhost;
	#access_log logs/host.access.log main;
	location ~ ^/FreeBSD:([[:digit:]]+):amd64/latest/(.+)$ {
	allow 10.0.0.0/24;
	deny all;
	autoindex on;
	alias /usr/local/poudriere/data/packages/$1amd64-default/$2;
	}

Ref: https://forums.freebsd.org/threads/local-cache-for-pkgs.60859/post-365296

[skhal]

nginx caching mechanism:
https://docs.nginx.com/nginx/admin-guide/content-cache/content-caching/

[skhal]

Store PKG cache in a separate ZFS dataset to separate data from the jail.