add a standalone host

1. install docker. the pi uses docker's own apt repo, and truenas and synology ship it

2. start the agent:

   docker run -d -p 9001:9001 --name portainer_agent --restart=always \
     -v /var/run/docker.sock:/var/run/docker.sock \
     -v /var/lib/docker/volumes:/var/lib/docker/volumes \
     -v /:/host:ro \
     portainer/agent:2.45.1
   

   • keep the agent on the same version as the server
   • on synology, docker's data is under /volume1/@docker, so mount /volume1/@docker/volumes there instead, see syn02
   • on truenas the agent runs as a truenas custom app instead, see truenas1

3. in portainer, add a docker standalone environment using the agent, at <host>:9001

4. add its stacks from git as on the swarm: reference refs/heads/deploy/<host>/<stack>, compose path stacks/<host>/<stack>/compose.yml