build base images periodically for test assembly

[joepvd]

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[openshift-ci]

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[openshift-ci]

@joepvd: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
ci/prow/security	0082fe0	link	false	/test security

Full PR test history. Your PR dashboard.

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