Fix resolve dockerfile name

[thegreyd]

Right now images:streams prs open breaks because of this.
We don't need the absolute path of Dockerfile but just the filename,
so we do not need to pass in source_path.

[openshift-ci]

@thegreyd: 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	38f5425	link	false	/test security

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[ashwindasr]

/lgtm

test rebase succeeded as well, on local

[thegreyd]

/approve

[openshift-ci]

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