Create Docker Backend to support Local Execution

[briangallagher]

What you would like to be added?

Currently there is a Kubernetes backend for full remote distributed training and SubProcess backend for Local execution.
The proposal is to add an additional backend for Docker, also to be used in a Local context.

Why is this needed?

This bridges the gap between the current subprocess backend and a full kubernetes backend by facilitating training in a containerised environment and across multiple containers.

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[briangallagher]

/assign