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>
diff --git a/scripts/build-unit-test-docker b/scripts/build-unit-test-docker
index a25f747..d4af404 100755
--- a/scripts/build-unit-test-docker
+++ b/scripts/build-unit-test-docker
@@ -17,6 +17,9 @@
 #   UBUNTU_MIRROR:    <optional, the URL of a mirror of Ubuntu to override the
 #                     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)
 #   http_proxy        The HTTP address of the proxy server to connect to.
 #                     Default: "", proxy is not setup if this is not set
 
@@ -700,6 +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")
 http_proxy = os.environ.get("http_proxy")
 
 gerrit_project = os.environ.get("GERRIT_PROJECT")
@@ -765,7 +769,7 @@
 
 # Create base Dockerfile.
 dockerfile_base = f"""
-FROM {distro}
+FROM {docker_reg}/{distro}
 
 {mirror}