Now that there is no more race between the containers for
preparing the volume, we can use it for podman as well...

Previously we risked ending up with not copying the contents
of /var from the container image, which made it not work.

The volume is created as empty, so make sure that we run a
container to do the initial copy of /var from the image.

This is to avoid race conditions later on, where the preload
starts to use the created volume for packing up the tarball.

Previously hardcoded to *always* use a Docker volume,
even if using a different container driver (podman).

Move the "sudo" prefix to a central location, instead of
having it all over the place. Assume only needed on Linux.

Use sudo for the podman commands instead

Wrap the docker commands with env prefix