Feat: DNS service discovery before container is healthy

[truong-hua]

Description

Currently, until docker 20.10, only healthy containers are available to be discovered through Docker Swarm DNS. It's great for the use case that another service would like to access a ready service. But in the use case that the service would like to detect it's peers to form a new cluster. Because the cluster is in initializing state so it can not be healthy and can not serve other clients, but without reporting healthy containers, peers won't be able to discover each other via Swarm DNS.

Examples of service that need to discover peers to form a cluster: etcd, rabbitmq, bookkeeper...

Expectation:

There is another domain that can be used for discover peers even they are not healthy yet (ex: unhealthy-tasks.service-name). And this behaviour is also true for hostname discovery of #34244. It could be a prefix like "unhealthy-" or "any-" to all of available current DNS domains.