Drop --continue-at from curl call

It is incompatible with -J and done automatically. See [1].

[1] https://gitlab.archlinux.org/archlinux/archlinux-docker/-/jobs/34434#L72

Dockerfile.template

@@ -13,7 +13,7 @@ COPY TEMPLATE_ROOTFS_FILE /
 RUN apk add --no-cache curl bash
 SHELL ["/bin/bash", "-c"]
 # TEMPLATE_ROOTFS_RELEASE_URL
-RUN ROOTFS="$(curl -sOJL --continue-at - -w "%{filename_effective}" TEMPLATE_ROOTFS_URL)" && \
+RUN ROOTFS="$(curl -sOJL -w "%{filename_effective}" TEMPLATE_ROOTFS_URL)" && \
     sha256sum -c <<< "TEMPLATE_ROOTFS_HASH" && \
     mkdir /rootfs && \
     tar -C /rootfs --extract --file "${ROOTFS}"