Dockerfile: Support multiple stage build in dlk and frontend

[YujiOshima]

Same as #25
/assign @gaocegege

[k8s-ci-robot]

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

Test name	Commit	Details	Rerun command
kubeflow-hp-tuning-presubmit	c6e3589	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]

/lgtm

[k8s-ci-robot]

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