The agent plugin can be configured to query specific instances and not to auto-detect them.
Use the agent bakery or put a file nginx_status.cfg
into /etc/check_mk
on your Docker node with the list of instances:
servers = [{'address': '127.0.0.1',
'page': 'nginx_status',
'port': 80,
'protocol': 'http'},
{'address': '172.16.32.12',
'page': 'nginx_status',
'port': 80,
'protocol': 'http'}]