test: add minimal test for container based building of images

This commit adds a smoke test that builds the ibcli container
and runs a fedora-41 raw-minimal build to double check that
the container based building actually works.

Thanks to Ondrej for suggesting this.

test/test_container.py

@@ -0,0 +1,27 @@
+import os
+import subprocess
+
+import pytest
+
+from containerbuild import build_container_fixture
+
+
+@pytest.mark.skipif(os.getuid() != 0, reason="needs root")
+def test_container_builds_image(tmp_path, build_container):
+    output_dir = tmp_path / "output"
+    output_dir.mkdir()
+    subprocess.check_call([
+        "podman", "run",
+        "--privileged",
+        "-v", f"{output_dir}:/output",
+        build_container,
+        "build",
+        "minimal-raw",
+        "--distro", "fedora-41"
+    ])
+    arch = "x86_64"
+    assert (output_dir / f"fedora-41-minimal-raw-{arch}/xz/disk.raw.xz").exists()
+    # XXX: ensure no other leftover dirs
+    dents = os.listdir(output_dir)
+    assert len(dents) == 1, f"too many dentries in output dir: {dents}"
+