Usage of containerId is shadowed from the user point of the view.
If you look at this code - https://github.com/apache/qpid-dispatch/blob/master/src/router_core/agent_config_auto_link.c#L413
you will see -

qd_parsed_field_t *in_use_conn = is_container ? container_field : connection_field;

which means if there is a container field, use it otherwise use the connection_field to specify the connection to use.

This is not specified in the documentation. It means that when user has created connector entity, assigned to the autoLink/linkRoute entity, this connector won't be used if containerId is specified.