warn on invalid Docker network_mode for Connect

[tgross]

A docker task with with Connect enabled has its network_mode implicitly set to container:<name of pause container> and users should not set the network_mode manually or their task will be in a different network namespace from Envoy. This is documented in #10724.

We should also have the Nomad client warn the user against doing this. We might not want to try to make it an error, as there may be some edge cases I haven't considered here. (Ex. can the sidecar_task have an explicit network_mode?)

[tgross]

This also impacts task-to-task network communication over localhost in general. The more I'm looking at this, I'm not sure it's ever a good idea to allow setting network_mode = "bridge" here?