--docker flag: enable agent2 docker plugin (socket access via docker group)

README.md

@@ -15,6 +15,7 @@ works behind NAT/firewalls and needs no inbound ports.
 # clone once, then git pull for updates:
 git clone https://gitea.noctools.nz/noctools/zabbix-agent.git
 sudo ./install-zabbix-agent2-ubuntu.sh --server <server-ip-or-name>
+sudo ./install-zabbix-agent2-ubuntu.sh --server <server-ip-or-name> --docker   # + docker monitoring
 ```
 
 Or with no local copy at all:
@@ -30,9 +31,18 @@ curl -fsSL https://gitea.noctools.nz/noctools/zabbix-agent/raw/branch/main/insta
 |---|---|---|
 | `--server HOST[:PORT]` | Zabbix server/proxy for active checks (**required**) | — |
 | `--hostname NAME` | Zabbix host name — must equal the technical name on the server | short system hostname |
+| `--docker` | enable the agent2 docker plugin: grants the `zabbix` user access to `/var/run/docker.sock` via the `docker` group (Docker must already be installed) | off |
 | `--major X.Y` | Zabbix repo series | `7.4` |
 
 After installing, create the host on the Zabbix server with the **exact same
 technical name** and an active-agent template (e.g. "Linux by Zabbix agent
 active"). The script reports whether the agent reached the server and warns
 if the host doesn't exist yet — the warning clears by itself once it does.
+
+## Docker monitoring
+
+`--docker` has the agent talk to `/var/run/docker.sock` (adds the `zabbix`
+user to the `docker` group). On the Zabbix server, link the host to a Docker
+template that uses **active** checks — e.g. "Docker by Zabbix agent 2 active"
+(where present) — since agents installed by this script are reached via
+active checks only.