Define and document requirements for the base image

Currently the base image does not have "deb-src" entries in /etc/apt/sources.list. I think this is a mistake and we should document that any base image should have source and binary repositories enabled in sources.list.

But even if it's not a mistake, we must document that base images should not have deb-src lines (and ensure that job images that do need them have a way to add them).

We could also document that:

• a base image has no package list, so "apt-get update" must be called before being able to perform any operation
• a base image has only the repositories associated to the target release in its sources.list