Switch to building with Depot

Change-Id: I4547e671e11f76e6776192cc68040d5e2ff61be9
diff --git a/images/heat/Dockerfile b/images/heat/Dockerfile
index 6d37b52..318c06f 100644
--- a/images/heat/Dockerfile
+++ b/images/heat/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: 2024-06-25T22:49:25Z
 
-ARG REGISTRY
-ARG RELEASE
-
-FROM ${REGISTRY}/openstack-venv-builder:${RELEASE} AS build
+FROM openstack-venv-builder AS build
 ARG HEAT_GIT_REF=80eea85194825773d1b60ecc4386b2d5ba52a066
 ADD --keep-git-dir=true https://opendev.org/openstack/heat.git#${HEAT_GIT_REF} /src/heat
 RUN git -C /src/heat fetch --unshallow
@@ -14,7 +12,7 @@
         /src/heat
 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 \