Provide the stage3 file via ARGS

My distfiles path on my host does not match the one mounted into the container.
Symlinks are not followed by podman, AFAICT due to security concerns.
Using ARGS to change the path seems reasonable to me.

Dockerfile

@@ -7,7 +7,8 @@
 
 FROM scratch as bootstrap
 # Unpack the stage3 archive (which is downloaded and verifed externally).
-ADD latest-stage3.tar.xz /
+ARG ROOTFS_FILENAME=""
+ADD ${ROOTFS_FILENAME} /
 # Set the profile.
 RUN eselect profile set "default/linux/amd64/17.1/no-multilib/systemd/merged-usr"
 # Replace /etc/portage/make.conf.

build-image.sh

@@ -91,6 +91,7 @@ podman tag rm "${REGISTRY}:latest" # Do not exit_err here. At least on first run
                                    # there is no latest tag to delete.
 
 # === Build the new image.
-podman build "${PODMAN_BUILD_ARGS[@]}" || exit_err "Build failed."
+podman build --build-arg=ROOTFS_FILENAME="${DISTFILES}/${ARCHIVE_FILE_NAME}" \
+  "${PODMAN_BUILD_ARGS[@]}" || exit_err "Build failed."
 
 # vim:fileencoding=utf-8:ts=4:syntax=bash:expandtab