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>
diff --git a/scripts/build-unit-test-docker b/scripts/build-unit-test-docker
index d4af404..628d9ab 100755
--- a/scripts/build-unit-test-docker
+++ b/scripts/build-unit-test-docker
@@ -18,8 +18,8 @@
 #                     default ones in /etc/apt/sources.list>
 #                     default is empty, and no mirror is used.
 #   DOCKER_REG:       <optional, the URL of a docker registry to utilize
-#                     instead of the default docker hub
-#                     (ex. public.ecr.aws/ubuntu)
+#                     instead of our default (public.ecr.aws/ubuntu)
+#                     (ex. docker.io)
 #   http_proxy        The HTTP address of the proxy server to connect to.
 #                     Default: "", proxy is not setup if this is not set
 
@@ -703,7 +703,7 @@
 distro = os.environ.get("DISTRO", "ubuntu:noble")
 branch = os.environ.get("BRANCH", "master")
 ubuntu_mirror = os.environ.get("UBUNTU_MIRROR")
-docker_reg = os.environ.get("DOCKER_REG", "docker.io")
+docker_reg = os.environ.get("DOCKER_REG", "public.ecr.aws/ubuntu")
 http_proxy = os.environ.get("http_proxy")
 
 gerrit_project = os.environ.get("GERRIT_PROJECT")