Airflow: DockerOperator support for Nvidia-Docker

[zacwellmer]

Description

Offer support for Nvidia-Docker. I'm not sure how feasible this is b/c it seems like docker's python package doesn't support the gpus flag (related issue w/ docker's python package). Although it does appear to be on their radar (this PR)

Use case / motivation
Support running GPU accelerated docker operators. This would also be useful to folks looking to run tasks in parallel on separate GPUs.

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[nullhack]

DockerOperator uses docker-py internally, so It'll not be possible to think about including It until that PR is merged.

[EpicWink]

PR merged!

[zacwellmer]

@nullhack could be a good time to revisit the issue now that the PR is merged