Ensure the postgres 12.X images from `Chart.yaml` annotations are packaged in releases

Summary

Looks like we need to populate a tests/images.txt to ensure postgresql 12.X images are included in releases. This looks to have been the case in releases from 2023.

see https://repo1.dso.mil/big-bang/product/packages/mattermost/-/blob/main/chart/Chart.yaml#L42