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/web-app-pretix/templates/docker-compose.yml.j2

@@ -1,8 +1,6 @@
 {% include 'roles/docker-compose/templates/base.yml.j2' %}
   application:
-    build:
-      context: .
-      dockerfile: Dockerfile
+{% include 'roles/docker-container/templates/build.yml.j2' %}
     image: "{{ PRETIX_IMAGE_CUSTOM }}"
     container_name: "{{ PRETIX_CONTAINER }}"
     hostname: '{{ PRETIX_HOSTNAME}}'