Improve caching documentation.

It was a little weak, and folk need enough detail to reason about
performance.

Change-Id: I67f81ab5a206b3d42dd1e4e1eb52af88ac1a3adc
Capture the repositories from source-repositories.

We try to expose metadata to users about what dib included in the
image and source-repositories was not doing that, so this fixes it. It
also permits easy introspection from other elements (such as an
upcoming PyPI cache layer).

Change-Id: I1e48e65f69ebe1f4556cc3bf35ccf195e99bcc76