docker: use Nomad managed resolv.conf when DNS options are set

[nickethier]

When network DNS configuration was initially added, the implementation plumbed the config into the docker driver's DNS related field. This caused issues when using bridge or cni Nomad networking modes as docker will fail to create the container because these DNS options cannot be set when using a non-default network mode. In this case Nomad's bridge networking mode sets the docker network mode to container.

This PR fixes this issue by reusing logic in exec and other drivers to manage the DNS configuration and mount it into the task. This way Nomad generates the resolv.conf file and mounts it into the task automatically.

fixes #8431

[shoenig]

LGTM!

[baxor]

Did this fix make it into the enterprise release? We are currently hitting this error (failed to create container: API error (400): conflicting options: dns and the network mode) with dns_servers and dns_search_domain set at the task level while testing consul connect/sidecar proxy. Running 1.1.9+ent

[tgross]

@baxor yes, that'd be in enterprise as well. Can you open a new issue describing the problem you've got? Thanks.

[baxor]

DNS configuration and mount it into the task. This way Nomad generates the resolv.conf file and mounts it into the task autom

Thanks, I opened #11857 -- happy to provide any further details.

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