To download some files, set the proxies inside a RUN section that downloads them. Also launch podman build with the --net=host to connect from within the container. This should not change the later container's access in any way. It only applies during the RUN sections.

Another method is to download them before the build into the build context (host directory). Then COPY them into the container.

Which is simpler depends on your needs. The former if others need to run the Containerfile without a shared context/directory, the latter if you want to lock down the version of what's downloaded.