docker run -d -v /path/to/horde/storage:/etc/horde -p 80:80 combro2k/horde-webmail
This container assumes that it would be mapped to port 80, which isn't always the truth. I ran it like this
docker run -d -v /path/to/horde/storage:/etc/horde -p 9888:80 combro2k/horde-webmail
but when I go to the host
<host-ip>:9888 I get redirected to http://<host-ip>/services/portal/index.php which does not exist, notice the missing port.