Add support for Docker Checks

Hi!
Consul supports health checking an application running into a container (while the consul agent is installed on the host). To do so, Consul uses a sort of docker exec ${containerId} my-health-check.sh to determine if the application is healthy or not.

Here how the check looks in json format (taken from official documentation https://www.consul.io/docs/agent/checks.html) :

{
"check": {
    "id": "mem-util",
    "name": "Memory utilization",
    "docker_container_id": "f972c95ebf0e",
    "shell": "/bin/bash",
    "args": ["/usr/local/bin/check_mem.py"],
    "interval": "10s"
  }
}

It seems that spring-cloud-consul does not support this yet. Are you planning to add this feature ?

FYI Ecwid has already the necessary API, here is an example :

        NewCheck check= new NewCheck();
        check.setDockerContainerID(containerId);
        check.setShell(shell);
        check.setScript(script);

Thanks,

Start by locating the existing Spring Cloud Consul health-check configuration and its mapping to Consul's NewCheck API, then compare it with the Docker check example and the referenced Ecwid API. Done means Docker container ID, shell, and arguments can be represented and sent as a Consul check, with coverage for the new configuration.