Refactor: unify Docker build config via build.yml.j2 include

Replaced duplicated inline build definitions in multiple docker-compose.yml.j2
templates with a shared include (roles/docker-container/templates/build.yml.j2).
This ensures consistent use of pull_policy: never and Dockerfile context across
services (Postgres, Bookwyrm, Bridgy Fed, Chess, Confluence, Jira, Moodle,
OpenProject, Pretix, Roulette Wheel, WordPress, XWiki, Simpleicons).

Conversation: https://chatgpt.com/share/68cd8f35-b764-800f-9b00-2c837103d2fb

roles/svc-db-postgres/templates/docker-compose.yml.j2

@@ -3,10 +3,7 @@
   postgres:
     container_name:   "{{ POSTGRES_CONTAINER }}"
     image:            "{{ POSTGRES_CUSTOM_IMAGE_NAME }}"
-    build:
-      context: .
-      dockerfile: Dockerfile
-    pull_policy: never
+{% include 'roles/docker-container/templates/build.yml.j2' %}
     command:
       - "postgres"
       - "-c"