This name is a better fit as it puts more emphasis on the format of the
resulting tree, not as much on the actual container engine used (which
can be swapped, and currently is in the CI where we use docker instead
of podman).
Also drop the awkward .docker symlink now and instead pass the container
engine through the MKTREE_ENGINE environment variable.
Turns out the docker command in CI is set up such that it doesn't need
an explicit sudo, so remove it. This will also allow us to easily pass
environment variables to mktree in the next commit (as otherwise we'd
have to use the ugly -E, --preserve-env option).
No functional change.
Michal Domonkos