Mount host log directory into containers for on-host syslog file

Previously --log-file only set DECNET_LOG_FILE inside containers with no
volume mount, so logs were lost on container restart and never visible on
the host. Now generate_compose binds the host log dir to
/var/log/decnet inside every service container so the file appears on
the host as expected.

- composer.py: _resolve_log_file() maps host path → container path;
  generate_compose() mkdir's host dir and injects volume + DECNET_LOG_FILE
- tests/test_log_file_mount.py: 9 new tests covering mount format,
  env injection, dir creation, and dedup

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/test_log_file_mount.py

@@ -0,0 +1,97 @@
+"""Tests for log_file volume mount in compose generation."""
+
+from pathlib import Path
+
+import pytest
+
+from decnet.composer import _CONTAINER_LOG_DIR, _resolve_log_file, generate_compose
+from decnet.config import DeckyConfig, DecnetConfig
+from decnet.distros import DISTROS
+
+
+def _make_config(log_file: str | None = None) -> DecnetConfig:
+    profile = DISTROS["debian"]
+    decky = DeckyConfig(
+        name="decky-01",
+        ip="10.0.0.10",
+        services=["http"],
+        distro="debian",
+        base_image=profile.image,
+        build_base=profile.build_base,
+        hostname="test-host",
+    )
+    return DecnetConfig(
+        mode="unihost",
+        interface="eth0",
+        subnet="10.0.0.0/24",
+        gateway="10.0.0.1",
+        deckies=[decky],
+        log_file=log_file,
+    )
+
+
+class TestResolveLogFile:
+    def test_absolute_path(self, tmp_path):
+        log_path = str(tmp_path / "decnet.log")
+        host_dir, container_path = _resolve_log_file(log_path)
+        assert host_dir == str(tmp_path)
+        assert container_path == f"{_CONTAINER_LOG_DIR}/decnet.log"
+
+    def test_relative_path_resolves_to_absolute(self):
+        host_dir, container_path = _resolve_log_file("decnet.log")
+        assert Path(host_dir).is_absolute()
+        assert container_path == f"{_CONTAINER_LOG_DIR}/decnet.log"
+
+    def test_nested_filename_preserved(self, tmp_path):
+        log_path = str(tmp_path / "logs" / "honeypot.log")
+        _, container_path = _resolve_log_file(log_path)
+        assert container_path.endswith("honeypot.log")
+
+
+class TestComposeLogFileMount:
+    def test_no_log_file_no_volume(self):
+        config = _make_config(log_file=None)
+        compose = generate_compose(config)
+        fragment = compose["services"]["decky-01-http"]
+        assert "DECNET_LOG_FILE" not in fragment.get("environment", {})
+        volumes = fragment.get("volumes", [])
+        assert not any(_CONTAINER_LOG_DIR in v for v in volumes)
+
+    def test_log_file_sets_env_var(self, tmp_path):
+        config = _make_config(log_file=str(tmp_path / "decnet.log"))
+        compose = generate_compose(config)
+        fragment = compose["services"]["decky-01-http"]
+        env = fragment["environment"]
+        assert "DECNET_LOG_FILE" in env
+        assert env["DECNET_LOG_FILE"].startswith(_CONTAINER_LOG_DIR)
+        assert env["DECNET_LOG_FILE"].endswith("decnet.log")
+
+    def test_log_file_adds_volume_mount(self, tmp_path):
+        config = _make_config(log_file=str(tmp_path / "decnet.log"))
+        compose = generate_compose(config)
+        fragment = compose["services"]["decky-01-http"]
+        volumes = fragment.get("volumes", [])
+        assert any(_CONTAINER_LOG_DIR in v for v in volumes)
+
+    def test_volume_mount_format(self, tmp_path):
+        config = _make_config(log_file=str(tmp_path / "decnet.log"))
+        compose = generate_compose(config)
+        fragment = compose["services"]["decky-01-http"]
+        mount = next(v for v in fragment["volumes"] if _CONTAINER_LOG_DIR in v)
+        host_part, container_part = mount.split(":")
+        assert Path(host_part).is_absolute()
+        assert container_part == _CONTAINER_LOG_DIR
+
+    def test_host_log_dir_created(self, tmp_path):
+        log_dir = tmp_path / "newdir"
+        config = _make_config(log_file=str(log_dir / "decnet.log"))
+        generate_compose(config)
+        assert log_dir.exists()
+
+    def test_volume_not_duplicated(self, tmp_path):
+        """Same mount must not appear twice even if fragment already has volumes."""
+        config = _make_config(log_file=str(tmp_path / "decnet.log"))
+        compose = generate_compose(config)
+        fragment = compose["services"]["decky-01-http"]
+        log_mounts = [v for v in fragment["volumes"] if _CONTAINER_LOG_DIR in v]
+        assert len(log_mounts) == 1