Code review comment for lp:~paelzer/curtin/bug-1574113-derived-repositories

This is really nice.

A couple of nits in the code.

Super happy to see unittests AND vmtests +1

One last request, might be a doc/topics/apt_source.rst

I think most of it can be copied out of your example files which included lots of
documentation, but in general, I'd like to see some documentation that we'd hand
to the MAAS team about how to use the new dictionary config to pass through the
landscape derived repo.