Skip tarball generation for layers when building a container image

[meeuw]

I'm using buildah to build large (20GB) images, I'm not interested in pushing the image to a registry because I'm using it to build a squashfs image from the contents.

I like the intermediate layers when building an image which allow me to restart at a failing step (skipping all steps that completed successfully).

But as the image increases in size during build, the layers grow as well and a tarball is generated for every layer in $TMPDIR which requires a lot of disk space, wasted time and wear on my SSD.

Currently I'm working around this by using a shell script instead of a Containerfile which uses the following shell script:

#!/bin/bash
set -eux
SRC=$1
DST=$2
buildah from --name=$DST scratch
buildah unshare << @EOF
set -eu
SRC_MOUNT=\$(buildah mount $SRC)
DST_MOUNT=\$(buildah mount $DST)
shopt -s dotglob 
cp --reflink=always -rp \${SRC_MOUNT}/* \${DST_MOUNT}/
@EOF

To create a reflinked clone of a buildah container. But this only works on btrfs (and zfs?).

This leads me to two questions:

1. Would it be possible to disable this tarball generation when using a Containerfile, with the side note that the generated image cannot be pushed?
2. Would it be a good idea to add a buildah clone command for this? There's already podman clone but that creates a new container from the source image discarding all changes from the source container.