For whatever reason, using the docker UI in synology to set up Portainer was not working for me. It would run, but it wouldn’t connect to my endpoint. Finally I found that if I ran the docker command from the CLI it would work.
So, to get portainer running, log in via SSH to your synology and sudo to su.
$ sudo su -
Then run the following docker command:
$ docker run -d -p 8000:8000 -p 9000:9000 --name=portainer --restart=always -v /var/run/docker.sock:/var/run/docker.sock -v /volume1/docker/portainer:/data portainer/portainer:latest
Updating the image also a problem. If you use the UI it will have the same issue of not connectiing to the endpoint. So it is better to just delete the container and run through the steps above again if you want to update.