test: container creation script with /dev/null

Add a script that creates a container image as an oci-archive that
contains /dev/null.

test/data/scripts/mkcontainerdevnull.sh

@@ -0,0 +1,27 @@
+#!/usr/bin/bash
+#
+# Create a container as an oci-archive that contains /dev/null.
+#
+# Used to test embedding containers that contain device nodes in ostree commits
+# (https://github.com/coreos/rpm-ostree/pull/5114).
+#
+# Requires root to create the node and to copy it into the container image.
+
+set -euxo pipefail
+
+tmpdir=$(mktemp -d -p .)
+
+cleanup() {
+    rm -r "${tmpdir}"
+}
+trap cleanup EXIT
+
+echo "This is a simple container that contains /dev/null for testing rpm-ostree commit creation." > "${tmpdir}/README"
+
+mkdir "${tmpdir}/dev"
+mknod -m 666 "${tmpdir}/dev/null" c 1 3
+
+amd=$(buildah from --arch=amd64 scratch)
+buildah config --created-by "Achilleas Koutsou" "${amd}"
+buildah copy "${amd}" "${tmpdir}" /
+buildah commit --format=oci --rm "${amd}" oci-archive:container-with-devnull.tar