In case the container exposes the SSH port on 0.0.0.0 and the docker daemon is running on a unix socket, the plugin reads the NetworkSettings.IPAddress field from the docker container to determine what host to connect to in order to talk to the container. However, the field is only filled when the container runs with the default "bridge" network. If another network is used (e.g., if a custom network is configured in the "Network" field of the docker template setting) this field is empty and the field NetworkSettings.<network name>.IPAddress has to be used. (cf. https://github.com/docker/docker/issues/21658#issuecomment-203527083).