In Container Linux, Docker Cloud has problems reconnecting to "bring your own Swarms" (BYOS) when Swarm managers restart.
The issue seems have something to do with the dockercloud/server-proxy
container running on each manager.
A Docker Cloud shows that the Swarm is "unreachable":
UNREACHABLE The swarm is sending heartbeat pings and Docker Cloud is receiving them, but Cloud cannot connect to the swarm.
Things we know (that may or may not be important):