Document building a testenv image.

Change-Id: Iaca931a570ec00dd52ce75eaebf9a396d79c4c86
local-config isn't needed for te-workers.

So don't suggest using it - folk can add it if they want.

Change-Id: Ic0578dca4fe721c18c972b75fc1eb3ae01d4707d
Merge "Document building a testenv image."
Merge "local-config isn't needed for te-workers."