docker: include vanilla debian postgres client (#10269)

## Problem

We are chasing down segfaults in the storage controller
neondatabase/cloud#21010

This is for use by the storage controller, which links dynamically with
`libpq`. We currently use the neon-built libpq, but this may be unsafe
for use from multi-threaded programs like the controller, as it uses a
statically linked openssl

Precursor to #10258

## Summary of changes

- Include `postgresql-15` in container builds.

The reason for using version 15 is simply because that is what's
available in Debian 12 without adding any extra repositories, and we
don't have any special need for latest version in our libpq usage.

Dockerfile

@@ -69,6 +69,8 @@ RUN set -e \
         libreadline-dev \
         libseccomp-dev \
         ca-certificates \
+	# System postgres for use with client libraries (e.g. in storage controller)
+        postgresql-15 \
     && rm -rf /var/lib/apt/lists/* /tmp/* /var/tmp/* \
     && useradd -d /data neon \
     && chown -R neon:neon /data