No docs on how to take advantage of maas-proxy to cache custom images (e.g. for LXC/KVM containers)

MAAS now uses a full-featured squid3 proxy, rather than just a squid-deb-proxy. It will be really useful to include a section in the docs to describe what steps are needed to configure maas-proxy as a (transparent) caching proxy. This will for example greatly speed-up Juju deployments with LXC or KVM containers, as MAAS can cache the needed images, so multiple re-deployments (i.e. bootstrap+destroy-environment) on the same MAAS will take less time initially to get the images.

For known locations that support both HTTP and HTTPS (e.g. cloud-images.ubuntu.com) a clever transparent proxy will rewrite the URL from HTTPS to HTTP, so the content can be fetched and cached.