Use PostgreSQL 13 in bullseye

This is the distribution’s default PostgreSQL version, and all my
packages for that distribution expect—and are tested against—that
version.

postgres/Dockerfile

@@ -1,4 +1,4 @@
-FROM postgres:14-bullseye
+FROM postgres:13-bullseye
 
 RUN set -eux \
 	&& apt-get update \