Add hostname parameter to Docker driver

[carlosdp]

This adds the hostname parameter to the Docker driver. This is useful because some service discovery packages, such as Weave, use the hostname to group together containers into a service (in lieu of using the container names, which are unique per host).

[diptanu]

@carlosdp Won't this result into containers having the same name for TaskGroups with count more than 1? I think you would want the host name to be uniq in the cluster too?

[carlosdp]

Not necessarily. when using something like weave, having the same hostname means that container is addressable at that DNS address. So all the postgres containers have the same postgres.weave.local hostname and can be reached on a weave network via DNS as postgres, for example. http://docs.weave.works/weave/latest_release/weavedns.html#usage

[diptanu]

I see. Can we please make this clear in the docs that setting this would result in all Containers of the Task groups to have the same host name?

[diptanu]

This LGTM, @dadgar and @cbednarski any thoughts?

[carlosdp]

@diptanu done!

[diptanu]

@carlosdp Thanks!

[dadgar]

LGTM

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