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/vars/main.yml

@@ -16,7 +16,7 @@ POSTGRES_SUBNET:                          "{{ networks.local['svc-db-postgres'].
 POSTGRES_PASSWORD:                        "{{ applications | get_app_conf(application_id, 'credentials.POSTGRES_PASSWORD') }}"
 POSTGRES_PORT:                            "{{ database_port | default(ports.localhost.database[ application_id ]) }}"
 POSTGRES_EXPOSE_LOCAL:                    True # Exposes the db to localhost, almost everytime neccessary
-POSTGRES_CUSTOM_IMAGE_NAME:               "postgres_custom"
+POSTGRES_CUSTOM_IMAGE:                    "postgres_custom"
 POSTGRES_LOCAL_HOST:                      "127.0.0.1"
 POSTGRES_VECTOR_ENABLED:                  True # Required by discourse, propably in a later step it makes sense to define this as a configuration option in config/main.yml
 POSTGRES_RETRIES:                         5