test(docker,traefik): count header cells, not <thead; read cert days against the cert's own clock (#5253, #5254)

CI 8548: '<th' also matched '<thead', and the certificate test's clock was
read before its certificates were built, so 10 days came out as 10.00…

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/plugins/docker/test_docker_page.py

@@ -235,6 +235,6 @@ async def test_busiest_containers_show_the_number_with_its_scale(app):
     assert "Busiest: <strong>CI: Android · build</strong> on VDNT-Docker03" in page and "1.6 of 8 cores" in page
     assert ">1.6 cores<" in page and ">0.25 cores<" in page and ">820.0 MiB<" in page
     assert "--pct:20.25%" in page and ">—<" in page
-    assert page.count("<th") == 4
+    assert len(re.findall(r"<th[ >]", page)) == 4
     assert ">CI: Android · build<" in page        # the long CI name only as the hover title
     assert not hex_colours(page), hex_colours(page)