Specify distribution in postrgres Docker image

Debian 13 (Trixie) has become the default distribution for postgres
Docker images. However, we're using Debian 12 (Bookworm) for our Ruby
image. While it isn't strictly necessary, it makes sense to use the same
distribution in the two Docker images we depend on.

docker-compose.yml

@@ -4,7 +4,7 @@ services:
 
     # use the preferred version of the official Postgres image
     # see https://hub.docker.com/_/postgres/
-    image: postgres:13.16
+    image: postgres:13.16-bookworm
     environment:
       - POSTGRES_USER=postgres
       - POSTGRES_PASSWORD=$POSTGRES_PASSWORD