chore: can pass a db user as parameter

roles/_app_backup/templates/app_backup_postgres_docker.j2

@@ -4,6 +4,6 @@ when = everyday at {{ backup_app_service_conf_time | mandatory }}
 cd {{ backup_prod_dir }}/{{ app_instance_id }}
 rm {{ backup_prod_dir }}/{{ app_instance_id }}/{{ database_docker_name }}.sql
 chmod 777 {{ backup_prod_dir }}/{{ app_instance_id }}
-/usr/bin/docker run --rm --link {{ database_docker_name }}:db --net {{ database_docker_network }} -v {{ backup_prod_dir }}/{{ app_instance_id }}:/backup -e PGPASSWORD={{ database_password }} {{ postgres_docker_image }} pg_dumpall -h db -U postgres -f /backup/{{ database_docker_name }}_all.sql
+/usr/bin/docker run --rm --link {{ database_docker_name }}:db --net {{ database_docker_network }} -v {{ backup_prod_dir }}/{{ app_instance_id }}:/backup -e PGPASSWORD={{ database_password }} {{ postgres_docker_image }} pg_dumpall -h db -U {{ database_user | default("postgres") }} -f /backup/{{ database_docker_name }}_all.sql
 
 rc=$?; if [[ $rc != 0 ]]; then fatal "sql" ; fi
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