Refactor MariaDB and PostgreSQL roles for Ansible 2.20 compatibility and Infinito.Nexus conventions

- Replace legacy docker_container-based MariaDB deployment with docker-compose based workflow
- Add custom Dockerfile and docker-compose templates for MariaDB
- Split MariaDB command into separate arguments to avoid entrypoint parsing errors
- Introduce MARIADB_CUSTOM_IMAGE and MARIADB_EXPOSE_LOCAL variables
- Add docker_compose_flush_handlers to ensure correct handler execution on first run
- Replace utils/once/finalize.yml with utils/once/flag.yml for new run-once semantics
- Align variable naming with Infinito.Nexus UPPERCASE conventions
- Fix PostgreSQL custom image variable name (POSTGRES_CUSTOM_IMAGE_NAME → POSTGRES_CUSTOM_IMAGE)
- Remove obsolete flush_handlers var injection in svc-db-postgres/tasks/main.yml
- General cleanup after migration from Ansible 2.18 → 2.20

Conversation reference:
https://chatgpt.com/share/69306c81-9934-800f-b317-f53a8f246a73

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

@@ -2,7 +2,7 @@
 
   postgres:
     container_name:   "{{ POSTGRES_CONTAINER }}"
-    image:            "{{ POSTGRES_CUSTOM_IMAGE_NAME }}"
+    image:            "{{ POSTGRES_CUSTOM_IMAGE }}"
     {{ lookup('template', 'roles/docker-container/templates/build.yml.j2') | indent(4) }}
     command:
       - "postgres"