Just for info, I have HASS.IO in Docker on a Ubuntu Hyper-V Virtual Machine, with APCUPSd running outside of docker on the local machine.
I switched over from Home Assistant on a Python virtual environment, to HASS.IO in docker and tried korylprince’s apcupsd Hass.io add-on but it not compatible with ethernet connected UPS’s so I had to move back to the locally hosted apcupsd.
It would not work for me to start with, as it’s configuration directives used by the Network Information Server was set to permit only local connections.
Setting NISIP 0.0.0.0 in the apcupsd configuration file fixed the issue.