docker: specify registry to simplify Podman builds (#24607)

.devops/cpu.Dockerfile

@@ -3,7 +3,7 @@ ARG BUILD_DATE=N/A
 ARG APP_VERSION=N/A
 ARG APP_REVISION=N/A
 
-FROM ubuntu:$UBUNTU_VERSION AS build
+FROM docker.io/ubuntu:$UBUNTU_VERSION AS build
 
 ARG TARGETARCH
 
@@ -37,7 +37,7 @@ RUN mkdir -p /app/full \
     && cp .devops/tools.sh /app/full/tools.sh
 
 ## Base image
-FROM ubuntu:$UBUNTU_VERSION AS base
+FROM docker.io/ubuntu:$UBUNTU_VERSION AS base
 
 ARG BUILD_DATE=N/A
 ARG APP_VERSION=N/A