build-unit-test-docker: provide docker registry override

docker.io continues to tighten control[1] on access to its registry.
Provide a mechanism within this script to utilize a different registry
for the base ubuntu image.

[1]: https://www.docker.com/blog/november-2024-updated-plans-announcement/

Tested:
- Confirmed script default still works as expected
- Confirmed the following also works now:
  export DOCKER_REG=public.ecr.aws/ubuntu

Change-Id: I4e8251653003cfac514905f2d2a5e8787b8ec06a
Signed-off-by: Andrew Geissler <geissonator@yahoo.com>
1 file changed