Please consider merging branch apt-cacher-ng of repo https://git.spwhitton.name/propellor.

Sample text for changelog/description of changes:

* Add Apt.proxy property to set a host's apt proxy.
* Add Apt.useLocalCacher property to set up apt-cacher-ng.
* Rework Sbuild properties to use apt proxies/cachers instead of bind-mounting
  the host's apt cache.  This makes it possible to run more than one build at
  a time, and lets sbuild run even if apt's cache is locked by the host's apt.
  - If Apt.proxy is set, it is assumed that the proxy does some sort of
    caching, and sbuild chroots are set up to use the same proxy.
  - If Apt.proxy is not set, we install apt-cacher-ng, and point sbuild
    chroots at the local apt cacher.
  - Drop Sbuild.piupartsConfFor, Sbuild.piupartsConf, Sbuild.shareAptCache
    (API change)
    No longer needed now that we are using apt proxies/cachers.
  - Update sample config in haddock for Propellor.Property.Sbuild.
    Please compare both your config.hs and your ~/.sbuildrc against the haddock.

--spwhitton

merge done --Joey