Let Docker allocate the network subnet again

Reverts the explicit subnets: the pools were freed by deleting five networks, so
the network role is back to its single idempotent task - ensure the network
exists and Caddy is on it - and the Sites no longer declare a subnet. The
deployment readme keeps the note that a new Site needs a config, and therefore a
new tag, but no longer asks for a free /24.

If the pools fill up again the failure is recognisable: 'all predefined address
pools have been fully subnetted', and the choices are deleting stale networks or
declaring a subnet per Site.

ansible/readme.md

@@ -52,10 +52,9 @@ machine's `~/.ssh/config` says. (`ssh` on some machines complains about
 a local file-permission bug whose fix is `sudo chown -R root:root
 /etc/ssh/ssh_config.d`, not something this playbook works around.)
 
-Adding a Site is one line in `hosts`, a copy of `vars/hopsngrains.yml`, a
-`config/<site>.site.json`, and a **free subnet** — this host's Docker address
-pools are exhausted, so `network_subnet` (a free `/24` from `10.210.0.0/16`) is
-required and a network is only created with it.
+Adding a Site is one line in `hosts`, a copy of `vars/hopsngrains.yml`, and a
+`config/<site>.site.json`. The image is shared, so the config has to be in it:
+a new Site needs a new tag.
 
 ## Checking it landed