Introduce container_registry for image prefix

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Change-Id: I80067f53aa2b63286b654fff782f0f7b65a2d050
diff --git a/images/netoffload/Dockerfile b/images/netoffload/Dockerfile
index 2294305..66d1c8d 100644
--- a/images/netoffload/Dockerfile
+++ b/images/netoffload/Dockerfile
@@ -1,6 +1,7 @@
 # SPDX-License-Identifier: Apache-2.0
 # Atmosphere-Rebuild-Time: 2024-06-25T13:53:44Z
 
+ARG REGISTRY
 ARG RELEASE
 
 FROM harbor.atmosphere.dev/docker.io/library/golang:1.20 AS build
@@ -9,7 +10,7 @@
 WORKDIR /src
 RUN go build -v -o offloadctl ./cmd/offloadctl/main.go
 
-FROM harbor.atmosphere.dev/library/ubuntu:${RELEASE}
+FROM ${REGISTRY}/ubuntu:${RELEASE}
 RUN <<EOF bash -xe
 apt-get update -qq
 apt-get install -qq -y --no-install-recommends \