fix(self-host): pin the compose Postgres image by version and digest - #606

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Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: contributor-assistant/github-action@ca4a40a7d1004f18d9960b404b97e5f30a505a08. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/