change default version from `13` to `stable-20260824`

[[ -v ${CONTAINERD_IMAGE_NAME} ]] || CONTAINERD_IMAGE_NAME=base-runner

[[ -v ${CONTAINERD_CMD} ]] || CONTAINERD_CMD=buildah

[[ -v ${CONTAINERD_BUILD_FORMAT} ]] || CONTAINERD_BUILD_FORMAT=docker

[[ -v ${DEBIAN_VERSION} ]] || DEBIAN_VERSION=13

[[ -v ${DEBIAN_VERSION} ]] || DEBIAN_VERSION=stable-20260824

if [[ -v ${CONTAINERD_DEBUG} ]]; then

  SKOPEO_OPTS="${SKOPEO_OPTS} --debug"