Fix: Add missing runtime dependency postgresql-client for container image

The `postgresql-client` package is required to provide the `psql`
executable. Missed this package when streamlining the packages for being
available on Debian oldstable, stable and testing. The `postgres-client`
is a provides by the actual available versioned postgres client package
e.g. on stable `postgresql-client-15`.

Author: bjoernricks

.github/runtime-dependencies.list

@@ -64,6 +64,7 @@ libpq5
 msmtp
 msmtp-mta
 openssh-client
+postgresql-client
 postgresql-client-common
 python3
 rpm