Introduce workflows for building notebook-server images upon changes

[apo-ger]

This PR resolves part of: #6766

Introduce GitHub Action workflows for testing that notebook server images can be built for each PR.

[google-oss-prow]

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[kimwnasptd]

@apo-ger the changes look good. Only have one suggestion; should we also trigger these when the releasing/version/VERSION file changes? Right now IIUC these workflows won't run once we update the manifests for a new release #6975

[apo-ger]

@kimwnasptd Yes you are right, I updated the PR.

[thesuperzapper]

This was fixed as part of #7357.