It has 2 different receivers configured: logstash and slack.
You need to provide a valid slack api key and logstash endpoint. Both can be passed to the container as an environmental variable. For example:
docker run \
-e "SLACK_API=https://hooks.slack.com/services/MY/API/KEY" \
-e "LOGSTASH_URL=http://logstash:8080/" \
basi/alertmanager