Introduce container_registry for image prefix

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Change-Id: I80067f53aa2b63286b654fff782f0f7b65a2d050
diff --git a/images/ubuntu-cloud-archive/Dockerfile b/images/ubuntu-cloud-archive/Dockerfile
index 65550c4..5612548 100644
--- a/images/ubuntu-cloud-archive/Dockerfile
+++ b/images/ubuntu-cloud-archive/Dockerfile
@@ -1,9 +1,10 @@
 # SPDX-License-Identifier: Apache-2.0
 # Atmosphere-Rebuild-Time: 2024-06-25T13:53:44Z
 
+ARG REGISTRY
 ARG RELEASE
 
-FROM harbor.atmosphere.dev/library/ubuntu:${RELEASE}
+FROM ${REGISTRY}/ubuntu:${RELEASE}
 COPY trusted.gpg.d/ubuntu-cloud-keyring.gpg /etc/apt/trusted.gpg.d/ubuntu-cloud-keyring.gpg
 COPY <<EOF /etc/apt/sources.list.d/cloudarchive.list
 deb http://ubuntu-cloud.archive.canonical.com/ubuntu jammy-updates/bobcat main