use apt-cacher proxy to make building faster

[hyperknot]

It's possible and easy to set up a local apt proxy using apt-cacher. This would make the build process much faster as well as would be friendlier to the official raspbian repo server.
https://help.ubuntu.com/community/Apt-Cacher-Server

[skarbat]

@zsero I had a look at this a few months back and I found the mirror to occupy about 150GB or something... There is a way to actually clone only the needed packages by using debootstrap --download-only but this requires minor tuning on Spindle, there might probably be other ways too.

[pazdera]

This is related more to our environment than dr itself. If you want to set it up, feel free. It's going to save some time, but I personally think it's not going to be a massive change.

[hyperknot]

I know it's not going to be a massive change, but it's definitely one of the options we have to make building faster. I believe it would cut almost one minute from each building time.

@skarbat I think making a mirror would be huge, but apt-cacher would only be a proxy, not a mirror, but I'm not an expert in this.

[alex5imon]

guys, let's get back to this conversation after v.1.0.3 is released please.