fix: resolve host IP from Docker bridge network for Rancher Desktop compatibility (fixes #561)

`--add-host host.docker.internal:host-gateway` works on Docker Desktop
and standard Docker Engine but not on Rancher Desktop, where the
`host-gateway` keyword is not recognised.

Introduce `_get_host_ip()` which reads the gateway of the Docker bridge
network (e.g. 172.17.0.1) — a portable, concrete IP that works across
Docker Desktop, Docker Engine, and Rancher Desktop.  If the network
inspect fails for any reason the function falls back to `host-gateway`,
preserving the existing behaviour for standard installations.

Also add unit tests covering the success path, subprocess failure, and
empty-output fallback.

cli/planoai/docker_cli.py

@@ -40,6 +40,35 @@ def docker_remove_container(container: str) -> str:
     return result.returncode
 
 
+def _get_host_ip() -> str:
+    """Resolve the host machine's IP address reachable from inside Docker containers.
+
+    Docker Desktop (Mac/Windows) supports the ``host-gateway`` special keyword, but
+    Rancher Desktop and some Linux configurations do not.  As a more portable
+    alternative we read the gateway of the default Docker bridge network, which is
+    the same IP that ``host-gateway`` would resolve to on standard installations.
+
+    Falls back to ``host-gateway`` so existing Docker Desktop setups are unaffected.
+    """
+    result = subprocess.run(
+        [
+            "docker",
+            "network",
+            "inspect",
+            "bridge",
+            "--format",
+            "{{range .IPAM.Config}}{{.Gateway}}{{end}}",
+        ],
+        capture_output=True,
+        text=True,
+        check=False,
+    )
+    ip = result.stdout.strip()
+    if result.returncode == 0 and ip:
+        return ip
+    return "host-gateway"
+
+
 def _prepare_docker_config(plano_config_file: str) -> str:
     """Copy config to a temp file, replacing localhost with host.docker.internal.
 
@@ -88,6 +117,8 @@ def docker_start_plano_detached(
         item for volume in volume_mappings for item in ("-v", volume)
     ]
 
+    host_ip = _get_host_ip()
+
     options = [
         "docker",
         "run",
@@ -98,7 +129,7 @@ def docker_start_plano_detached(
         *volume_mappings_args,
         *env_args,
         "--add-host",
-        "host.docker.internal:host-gateway",
+        f"host.docker.internal:{host_ip}",
         PLANO_DOCKER_IMAGE,
     ]
 

cli/test/test_docker_cli.py

@@ -0,0 +1,46 @@
+import subprocess
+from unittest import mock
+
+from planoai.docker_cli import _get_host_ip
+
+
+def test_get_host_ip_returns_bridge_gateway():
+    """When docker network inspect succeeds, the bridge gateway IP is returned."""
+    fake_result = mock.Mock()
+    fake_result.returncode = 0
+    fake_result.stdout = "172.17.0.1\n"
+
+    with mock.patch("subprocess.run", return_value=fake_result) as mock_run:
+        ip = _get_host_ip()
+
+    assert ip == "172.17.0.1"
+    mock_run.assert_called_once()
+    args = mock_run.call_args[0][0]
+    assert "docker" in args
+    assert "network" in args
+    assert "inspect" in args
+    assert "bridge" in args
+
+
+def test_get_host_ip_falls_back_on_failure():
+    """When docker network inspect fails, 'host-gateway' is returned as a fallback."""
+    fake_result = mock.Mock()
+    fake_result.returncode = 1
+    fake_result.stdout = ""
+
+    with mock.patch("subprocess.run", return_value=fake_result):
+        ip = _get_host_ip()
+
+    assert ip == "host-gateway"
+
+
+def test_get_host_ip_falls_back_on_empty_output():
+    """When docker network inspect returns empty output, 'host-gateway' is returned."""
+    fake_result = mock.Mock()
+    fake_result.returncode = 0
+    fake_result.stdout = "   "
+
+    with mock.patch("subprocess.run", return_value=fake_result):
+        ip = _get_host_ip()
+
+    assert ip == "host-gateway"