Got it working now.
For anyone who stumbles upon this I had to install nano (my prefered editor)
apk add nano
Then run to open the groups
nano /etc/group
I then manually added to the group file. Your GID (281 below) may differ on your host machine.
docker:x:281:zabbix
Save and reboot docker container and it now works
Just note that your GID may differ from the one above (281). To find your docker group ID run the below on your host machine
getent group docker | awk -F: '{printf "Group %s with GID=%d\n", $1, $3}'
Thanks for the pointers @VRx