feat(docker): Docker's own status reaches Steward — an empty answer clears a host, a lost one greys it, a demoted manager's swarm rows go (#5152)

The agent (1.9.0) said nothing when Docker could not be asked and nothing
when Docker had no containers, so Steward could tell neither apart from the
other, and kept a host's last containers on screen as running either way.

- Agent: collect_docker returns (status, detail, containers). "ok" sends
  docker: [] when there are none; "unavailable" (no socket, nothing
  listening) and "error" (permission, timeout, bad answer) say why.
  collect_swarm says whether /info answered, and a node that is not a
  manager sends swarm_manager: false. A failed /info says nothing.
- Ingest: an empty docker list is a snapshot; the newest sample's status
  goes to the Docker plugin with the rest.
- docker_host_status (docker_011): per host, status, detail, since and
  last report. A host that never had Docker gets no row.
- An "ok" empty snapshot reaps the host's containers; an error keeps them.
  swarm_manager: false drops the host's swarm services and nodes.
- Docker page: a "Docker not answering" card names each such host, why,
  and since when; its containers read as stale. Duplicate header comment
  in rows.html removed.

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

tests/plugins/docker/test_docker_page.py

@@ -33,6 +33,9 @@ def test_stale_is_older_than_the_agents_window():
     assert not _is_stale(_c("a"), NOW, 180)
     assert _is_stale(_c("a", scraped_at=NOW - timedelta(minutes=10)), NOW, 180)
     assert _is_stale(_c("a", scraped_at=None), NOW, 180)
+    # A fresh row on a host whose Docker stopped answering is stale too (#5152).
+    assert _is_stale(_c("a"), NOW, 180, {"h1": object()})
+    assert not _is_stale(_c("a"), NOW, 180, {"h2": object()})
     assert svc.stale_after_seconds({"PLUGINS": {"host_agent": {"stale_after_seconds": 60}}}) == 60
     assert svc.stale_after_seconds({}) == 180
 
@@ -74,3 +77,29 @@ async def test_a_healthy_board_has_no_attention_card(app):
             app, "docker", "docker/rows.html", attention=[], says=svc.PROBLEM_SAYS, severity=svc.PROBLEM_SEVERITY,
             unhealthy=0, restarting=0, stale=0, host_groups=[], swarm_services=[], running=0, stopped=0, total=0)
     assert "Needs attention</h2>" not in page and "No containers reported yet" in page
+
+
+async def test_a_host_whose_docker_stopped_answering_says_so(app):
+    down = [{"host_id": "h1", "host_name": "nas", "status": "unavailable",
+             "detail": "no Docker socket at /var/run/docker.sock", "since": NOW - timedelta(minutes=5)},
+            {"host_id": "h2", "host_name": "web-1", "status": "error",
+             "detail": "permission denied on /var/run/docker.sock", "since": NOW - timedelta(minutes=1)}]
+    async with app.test_request_context("/plugins/docker/rows"):
+        page = await render_plugin_template(
+            app, "docker", "docker/rows.html", docker_down=down, attention=[], says=svc.PROBLEM_SAYS,
+            severity=svc.PROBLEM_SEVERITY, unhealthy=0, restarting=0, stale=0, host_groups=[],
+            swarm_services=[], running=0, stopped=0, total=0)
+    page = re.sub(r">\s+<", "><", " ".join(page.split()))
+    card = page[page.index("Docker not answering"):page.index("</section>")]
+    assert '<span class="badge badge-warn">No Docker</span><a href="/hosts/h1">nas</a>' in card
+    assert "no Docker socket at /var/run/docker.sock · since <time" in card and ">5m ago</time>" in card
+    assert '<span class="badge badge-crit">Error</span><a href="/hosts/h2">web-1</a>' in card
+
+
+async def test_no_card_while_every_docker_answers(app):
+    async with app.test_request_context("/plugins/docker/rows"):
+        page = await render_plugin_template(
+            app, "docker", "docker/rows.html", docker_down=[], attention=[], says=svc.PROBLEM_SAYS,
+            severity=svc.PROBLEM_SEVERITY, unhealthy=0, restarting=0, stale=0, host_groups=[],
+            swarm_services=[], running=0, stopped=0, total=0)
+    assert "Docker not answering" not in page