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.