fix(maintenance): check first, drain before a Docker upgrade, wait 5 min after a drain, see containers back (#6098)

On 2026-10-18 an update on VDNT-Docker02 restarted the Docker daemon six
seconds in, before the drain that came only with the reboot, and every
container on the node stopped. Docker's own packages restart the daemon from
their install scripts; needrestart's list-only mode doesn't stop that.

- The helper gains a `check` action (agent 1.24.0): a dry run of the update
  for each package manager, whether Docker is among it, and the running
  containers outside the swarm with their restart policies.
- A swarm node is drained before an update that upgrades Docker, else only
  before a reboot, and at most once a job.
- After a drain the job settles 5 minutes before the update or reboot.
- Once the work is over, the containers that ran outside the swarm must be
  seen running again within 10 minutes, or the job fails naming them.
- An older agent gets no check and its job runs as before.

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

plugins/host_agent/scheduler.py

@@ -147,8 +147,9 @@ def make_sensor_task(app) -> ScheduledTask:
 def make_maintenance_task(app) -> ScheduledTask:
     """Gives up on maintenance jobs past their deadline (#5767), starts due
     maintenance plans and moves their runs on one host at a time (#5768), and
-    records each host's maintenance_failed for an alert rule. Every move a job
-    itself makes comes from its agent's posts."""
+    records each host's maintenance_failed for an alert rule. A job's steps
+    move on from its agent's posts; the tick moves on only Steward's own waits
+    (the 5 minutes after a drain, the containers seen running again, #6098)."""
     async def _tick():
         from . import cadence, maintenance
         now = datetime.now(timezone.utc)