[Draft] Add GitHub Actions schedule to ensure images are refreshed every 2 weeks

[Porges]

Add a schedule trigger to the refresh workflow so that images are refreshed at least once every two weeks. This is the cadence that is documented in the wiki (1, 2).

As noted in several comments (1, 2), this cadence has not been met in the past.

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[Porges]

CC @manikantanallagatla

Note that this is not yet functional since at the moment the targetTag is derived from the custom event payload. However it could be derived directly from the GitHub tags present in the repository by selecting the latest one.

[davidstarkcab]

Is it possible for us to get an updated image every week? It would be really appreciated. It would also be appreciated if this PR (Adding dotnet:4-alpine image) could get some attention.