network: allow customisation of namespace hostname when using bridged networking

[jrasell]

When a job specification utilises network.mode = "bridge", all tasks within the group are placed in the same network namespace and therefore share a hostname. The hostname of the network namespace is currently set to the truncated allocation ID and there is no possibility to customise this.

Proposal

Allow operators to set custom namespace hostnames using fields within the job specification. If this parameter is not set, then the existing behaviour continues.

Detail

The creation of a network namespace is defined by the DriverNetworkManager interface. In the case of the Docker driver, the pause container is responsible for controlling the namespace hostname, otherwise the functionality is performed within the defaultNetworkManager. This is not currently supported within Windows environments.

This recent PR allowed for generation of /etc/hosts files when using bridged networking which could also provide a useful entry point in helping fix this problem.

[github-actions]

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