fix: update interface detection code to support additional interfaces Docker adds.

README.md

@@ -148,12 +148,17 @@ much the package reads from the system:
 |---|---|---|
 | `manual` | both interface names, `int_addr`, `int_gateway_ip` | nothing |
 | `interfaces` | both interface names | `int_addr` and `int_gateway_ip`, from those interfaces |
-| `auto` | nothing | the interface names too, from the routing table |
+| `auto` | nothing | the interface names too: the external one from the routing table, the internal one as the only other NIC with an address |
 
 `manual` is the only mode in which no detection code runs at all. `auto` is the
 convenient one and can pick the wrong interface, which is a trade you make deliberately
 by choosing it rather than something the package does behind your back.
 
+Detection ignores virtual interfaces, those the kernel lists under `/sys/devices/virtual`:
+loopback, bridges such as `docker0`, veth pairs and tunnels. Only the NICs of the machine
+count, so Docker does not change the result. If the host has more than two NICs with an
+address, `auto` stops and says so; use mode `interfaces` and name them.
+
 The addresses strongSwan binds on always come from the external NIC, in every mode, so
 they are never configured twice and cannot drift out of step with the host. If a named
 interface does not exist the service stops and says so. In `interfaces` and `auto` mode