provide docker registry override in remaining scripts

As done with fe2768c73, provide a mechanism to override the docker
registry used for our remaining docker scripts.

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 defaults still works as expected
- Confirmed the following also works now:
  export DOCKER_REG=public.ecr.aws/ubuntu
- Confirmed where applicable that this also works(fedora):
  export DOCKER_REG=public.ecr.aws/docker/library

Change-Id: Ib237183fb9f27573643783b3988cb00614749f4c
Signed-off-by: Andrew Geissler <geissonator@yahoo.com>
diff --git a/qemu-build.sh b/qemu-build.sh
index 3e5f422..54044e7 100755
--- a/qemu-build.sh
+++ b/qemu-build.sh
@@ -21,6 +21,9 @@
 #                     Default: "", proxy is not setup if this is not set
 #  WORKSPACE          Path of the workspace directory where the build will
 #                     occur, and output artifacts will be produced.
+#  DOCKER_REG:        <optional, the URL of a docker registry to utilize
+#                     instead of the default docker hub
+#                     (ex. public.ecr.aws/ubuntu)
 #
 ###############################################################################
 # Trace bash processing
@@ -34,6 +37,8 @@
     exit 1
 fi
 
+docker_reg=${DOCKER_REG:-"docker.io"}
+
 # Docker Image Build Variables:
 img_name=qemu-build
 
@@ -89,7 +94,7 @@
 # !!!
 
 Dockerfile=$(cat << EOF
-FROM ubuntu:jammy
+FROM ${docker_reg}/ubuntu:jammy
 
 ${PROXY}