build-unit-test-docker: make aws default docker registry

AWS has much better anonymous pull request limits (1 per second) vs
docker hub which is 100 per 6 hours per IP address. A lot of our CI and
development occurs on shared systems so it's easy to exceed this limit
on an IP basis.

Tested:
- Verified no setting for DOCKER_REG and a setting of docker.io work as
  expected

Change-Id: I7fb220b2541ccb1b65fa74b8638c0258a84b7421
Signed-off-by: Andrew Geissler <geissonator@yahoo.com>
1 file changed