Fix dockerfile processing behavior of cluster builder

[Minkyu-Choi]

What this PR does / why we need it: Set container command from preprocessor when using cluster builder

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged): Fixes #538

Special notes for your reviewer:

1. Please confirm that if this PR changes any image versions, then that's the sole change this PR makes.

Release note:

[kubeflow-bot]

This change is 

[k8s-ci-robot]

Hi @Minkyu-Choi. Thanks for your PR.

I'm waiting for a kubeflow member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[jinchihe]

/ok-to-test

[jinchihe]

/lgtm /approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: jinchihe

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