After installing the Bitnami PostgreSQL, the service is called postgresql and not coder-db-postgresql

So the Url should be:

postgres://coder:coder@postgresql.coder.svc.cluster.local:5432/coder?sslmode=disable

Than proceed with creating the secret and it works.

Docs link:
https://coder.com/docs/install/kubernetes#in-cluster-postgresql-for-proof-of-concepts

I recommend fixing quickly so anyone quickly trying out Coder on Kubernetes isn't wondering database isn't connecting to coder.