provider/docker: Add network_mode support to docker

[ColinHebert]

Fixes #2929

Add the possibility to customise the NetworkMode:
https://docs.docker.com/engine/reference/run/#network-settings
https://docs.docker.com/engine/reference/api/docker_remote_api_v1.21/#create-a-container
https://github.com/fsouza/go-dockerclient/blob/master/container.go

[ColinHebert]

@jen20 are we okay to proceed with this PR?

[jen20]

LGTM

[ghost]

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