maintenance: upgrade to postgres:17.7-trixie in docs and for Testcontainers (#222)

Co-authored-by: Michael Hoennig <michael@hoennig.de>
Reviewed-on: http://dev.hostsharing.net/hostsharing/hs.hsadmin.ng/pulls/222
Reviewed-by: Stefan Begerad <stefan.begerad@hostsharing.net>

etc/docker-compose.yml

@@ -2,7 +2,7 @@ version: '3.8'
 
 services:
   postgres:
-    image: postgres-with-contrib:15.5-bookworm
+    image: postgres-with-contrib:17.7-trixie
     container_name: custom-postgres
     environment:
       POSTGRES_PASSWORD: password