[Docker] Use `SELECT 1` to wait for database

This removes the need to have host and port as environment variables (we might get it in a YAML file from a K8s ConfigMap, or some such). Also, it ensures that the system answering is a database rather than an IoT Waffeleisen with a bad taste in port selection.

added part::backend part::ci part::packaging type::feature labels

added 1 commit

• a6d45dd5 - [Docker] Add psql to the image

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mentioned in commit 3b5b720d