Switch to building with Depot

Change-Id: I4547e671e11f76e6776192cc68040d5e2ff61be9
(cherry picked from commit 564caa8bd7f1cf771958a97b8fff7c9bef90e447)
diff --git a/images/ironic/Dockerfile b/images/ironic/Dockerfile
index df62bcc..21fc5f7 100644
--- a/images/ironic/Dockerfile
+++ b/images/ironic/Dockerfile
@@ -1,10 +1,8 @@
-# SPDX-License-Identifier: Apache-2.0
+# SPDX-FileCopyrightText: © 2025 VEXXHOST, Inc.
+# SPDX-License-Identifier: GPL-3.0-or-later
 # Atmosphere-Rebuild-Time: 2024-06-26T17:38:39Z
 
-ARG REGISTRY
-ARG RELEASE
-
-FROM ${REGISTRY}/openstack-venv-builder:${RELEASE} AS build
+FROM openstack-venv-builder AS build
 ARG IRONIC_GIT_REF=19de7ae2f48705cfa0e59d0642cec3cca7b6ca22
 ADD --keep-git-dir=true https://opendev.org/openstack/ironic.git#${IRONIC_GIT_REF} /src/ironic
 RUN git -C /src/ironic fetch --unshallow
@@ -16,7 +14,7 @@
         sushy
 EOF
 
-FROM ${REGISTRY}/openstack-python-runtime:${RELEASE}
+FROM openstack-python-runtime
 RUN <<EOF bash -xe
 apt-get update -qq
 apt-get install -qq -y --no-install-recommends \