run-builder: inspect network to get gateway IP

Instead of inferring the gateway ip via the the network device for
the podman bridge, where the name can change, use podman network
inspect.

```js
The json looks like this:
[
  {
    "cniVersion": "0.4.0",
    "name": "org.osbuild.koji",
    "plugins": [
      {
        "bridge": "cni-podman1",
        "hairpinMode": true,
        "ipMasq": true,
        "ipam": {
          "ranges": [
            [
              {
                "gateway": "10.89.0.1",
                "subnet": "10.89.0.0/24"
              }
            ]
          ],
          "routes": [
            {
              "dst": "0.0.0.0/0"
            }
          ],
          "type": "host-local"
        },
        "isGateway": true,
        "type": "bridge"
      },
      {
        "capabilities": {
          "portMappings": true
        },
        "type": "portmap"
      },
      {
        "backend": "",
        "type": "firewall"
      },
      {
        "domainName": "dns.podman",
        "type": "dnsname"
      }
    ]
  }
]
```

So the podman format for this is very obviously:

  {{ (index (index (index .plugins 0).ipam.ranges 0) 0).gateway }}

run-builder.sh

@@ -19,10 +19,13 @@ else
   exit 2
 fi
 
+GATEWAY_IP=$(podman network inspect org.osbuild.koji --format '{{ (index (index (index .plugins 0).ipam.ranges 0) 0).gateway }}')
+echo "Gateway IP is $GATEWAY_IP"
+
 ${CONTAINER_RUNTIME} run --rm -i -t --name org.osbuild.koji.builder --network org.osbuild.koji \
   -v "${SHARE_DIR}:/share:z" \
   -v "${DATA_DIR}:/mnt:z" \
   -v "${PWD}/container/builder/osbuild-koji.conf:/etc/koji-osbuild/builder.conf:z" \
   --hostname org.osbuild.koji.kojid \
-  --add-host=composer:$(ip route show dev cni-podman0 | cut -d\  -f7) \
+  --add-host=composer:${GATEWAY_IP} \
   koji.builder