Introduce container_registry for image prefix

Change-Id: I80067f53aa2b63286b654fff782f0f7b65a2d050
diff --git a/images/horizon/Dockerfile b/images/horizon/Dockerfile
index f9e6bbb..c51a3f7 100644
--- a/images/horizon/Dockerfile
+++ b/images/horizon/Dockerfile
@@ -1,9 +1,10 @@
 # SPDX-License-Identifier: Apache-2.0
 # Atmosphere-Rebuild-Time: 2024-06-25T22:49:25Z
 
+ARG REGISTRY
 ARG RELEASE
 
-FROM harbor.atmosphere.dev/library/openstack-venv-builder:${RELEASE} AS build
+FROM ${REGISTRY}/openstack-venv-builder:${RELEASE} AS build
 ARG HORIZON_GIT_REF=14212342cf8f7eb987e50de112958af31063e02e
 ADD --keep-git-dir=true https://opendev.org/openstack/horizon.git#${HORIZON_GIT_REF} /src/horizon
 RUN git -C /src/horizon fetch --unshallow
@@ -40,7 +41,7 @@
         pymemcache
 EOF
 
-FROM harbor.atmosphere.dev/library/openstack-python-runtime:${RELEASE}
+FROM ${REGISTRY}/openstack-python-runtime:${RELEASE}
 RUN <<EOF bash -xe
 apt-get update -qq
 apt-get install -qq -y --no-install-recommends \