This is a caching proxy, based on apt-cacher-ng.  

Config files are included, which will work out of the box for Debian,Ubuntu,Arch, and Fedora.  
The cache and log directories are bind-mounted in /rw in the cacher qube.

Copy directory to /srv/salt, then run:  
qubesctl state.apply cacher.create
qubesctl --skip-dom0 --targets=template-cacher state.apply cacher.install  
qubesctl --skip-dom0 --targets=cacher state.apply cacher.configure

To automatically use the proxy run:  
qubesctl state.apply cacher.use  
This will configure /etc/qubes/policy.d/30-user.policy to use the caching proxy by default.

apt-cacher-ng will cache HTTPS requests if you change https:// to http://HTTPS/// in repo source lists.
To configure all templates to use the proxy in this way, run:  
qubesctl --skip-dom0 --templates state.apply cacher.change_templates  
Or target individual templates, as you wish:
qubesctl --skip-dom0 --targets=TEMPLATE1,TEMPLATE2  state.apply cacher.change_templates  

N.B
apt-cacher-ng works well for Debian,Ubuntu,and Arch.  
It works reasonably well for Fedora, but may require further tweaking of the apt-cacher-ng control file, and the fedora_mirrors lists.

To restore a template to the default state, a state is provided, restore_templates.sls.
Apply it like this:
qubesctl --skip-dom0 --targets=TEMPLATE1,TEMPLATE2  state.apply cacher.restore_templates  

The qrexec policy file at  /etc/qubes/policy.d/30-user.policy should be edited so that these templates use the default system proxy.

Templates or qubes that cannot run over qrexec, can connect to the caching
server if you set cacher as netvm, or upstream, and configure the qube to use
Proxy address of CACHER_IP:8082.