test: add cross-build end-to-end test

This commit uses the new `build --arch=` support to perform
a cross arch build for aarch64, ppc64le, riscv64, s390x on
fedora. We could do also centos-9 each arch is already 20min.

test/test_container.py

@@ -96,3 +96,29 @@ def test_container_with_progress(tmp_path, build_fake_container, progress, needl
     ], text=True)
     assert needle in output
     assert forbidden not in output
+
+
+# only test a subset here to avoid overly long runtimes
+@pytest.mark.parametrize("arch", ["aarch64", "ppc64le", "riscv64", "s390x"])
+def test_container_cross_build(tmp_path, build_container, arch):
+    # this is only here to speed up builds by sharing downloaded stuff
+    # when this is run locally (we could cache via GH action though)
+    os.makedirs("/var/cache/image-builder/store", exist_ok=True)
+    output_dir = tmp_path / "output"
+    output_dir.mkdir()
+    subprocess.check_call([
+        "podman", "run",
+        "--privileged",
+        "-v", "/var/lib/containers/storage:/var/lib/containers/storage",
+        "-v", "/var/cache/image-builder/store:/var/cache/image-builder/store",
+        "-v", f"{output_dir}:/output",
+        build_container,
+        "build",
+        "--progress=verbose",
+        "--output-dir=/output",
+        "container",
+        "--distro", "fedora-41",
+        # selecting a foreign arch here automatically triggers a cross-build
+        f"--arch={arch}",
+    ], text=True)
+    assert os.path.exists(output_dir / f"fedora-41-container-{arch}.tar")