feat: Add docker backend

[briangallagher]

What this PR does / why we need it:
This PR adds a docker backend, extending the current Kubernetes and Local Process backends.
Its designed for Local Execution

Which issue(s) this PR fixes (optional, in Fixes #<issue number>, #<issue number>, ... format, will close the issue(s) when PR gets merged):

Fixes #108

Verification
This WIP PR to add an example notebook for local docker execution can be used to verify this PR
Update the pip install to:
!pip install "git+https://github.com/briangallagher/sdk.git@docker-backend#egg=kubeflow[docker]"

[google-oss-prow]

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[coveralls]

Pull Request Test Coverage Report for Build 18128142359

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 71.739%

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Totals
Change from base Build 17979146135:	0.0%
Covered Lines:	297
Relevant Lines:	414

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💛 - Coveralls

[astefanutti]

Suggested change

	DOCKER_LABEL_PREFIX = "trainer.kubeflow.ai"
	DOCKER_LABEL_PREFIX = "trainer.kubeflow.org"

[astefanutti]

How difficult would it be to support Podman as well: https://pypi.org/project/podman/

Should it be a different backend or could this be a "container" backend that could work for both Docker and Podman depending on what's installed / configured?

[briangallagher]

I think it's a little bit cleaner to have a separate backend as it uses the podman client and the code is a bit different. The networking is slightly different.
But, a nice addition might be some auto discovery after all backends are added.
Meaning, look for Podman, then docker finally fall back to subprocess.

[andreyvelich]

I would agree with @astefanutti to unify all container-based functionality under single backend.
From the end-user point of view it doesn't matter whether they use Docker, Podman, or Container backend to run local containers.