docker.mk: Add Podman support and fix volume/artifact handling

* Use `$(DOCKER)` for pull, build, run, images, and rmi so `DOCKER=podman` works
* Default `DOCKER` to 'docker' in docker.mk when unset
* Use localhost/ image prefix with Podman so run uses local image instead of pull
* Add `:z` volume option with Podman for SELinux bind-mount write access
* Create `ARTIFACTS_DIR` before container run to avoid missing directory errors

Signed-off-by: flouthoc <flouthoc.git@gmail.com>