Write package-cache to cache packages

[xiaom]

Iterate/Debug Dockerfile is a bit slow (and annoying) when you have to fetch all the packages from the mirrors. For the binary files that is not handled by apt, I download them locally. For the others, we may consider write some package-cache handling on-demand caching for Ubuntu, just like what @wking did for Gentoo.

[wking]

On Fri, Mar 13, 2015 at 08:56:20AM -0700, Xiao Meng wrote:

For the others, we may consider write some package-cache handling
on-demand caching for Ubuntu, just like what @wking did for
Gentoo.

I'm happy to extend package-cache to work with apt packages. I'm just
not familiar enough with the apt mirrors or API to do that myself. If
someone wants to explain how apt fetches packages, I can take a stab
at it. I'm also happy to just merge a PR that someone else works up
;).