Ubuntu provisionned: unable to resize disk image because of shrink

On my test, the one-context Ubuntu images does not build with the following error:

==> base-onecontext.qemu.ubuntu: Error creating hard drive: QemuImg error: qemu-img: Use the --shrink option to perform a shrink operation.
==> base-onecontext.qemu.ubuntu: qemu-img: warning: Shrinking an image will delete all data beyond the shrunken image's end. Before performing such an operation, make sure there is no important data there.