Dockerfile: Use multiple stage builds

[gaocegege]

• Reduce the time spent on building (80% faster in China)
• Reduce the size of the image （891 MB -> 16.7 MB (2% of the original image))

/assign @YujiOshima

[YujiOshima]

/lgtm

[k8s-ci-robot]

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This pull-request has been approved by: YujiOshima

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[k8s-ci-robot]

@gaocegege: The following test failed, say /retest to rerun them all:

Test name	Commit	Details	Rerun command
kubeflow-hp-tuning-presubmit	fa9fde0	link	/test kubeflow-hp-tuning-presubmit

Full PR test history. Your PR dashboard. Please help us cut down on flakes by linking to an open issue when you hit one in your PR.

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[gaocegege]

The CI takes too long time thus I merged it manually first.