docker: warn about low inotify/file-max and missing FUSE at connect

Add a read-only _check_host_readiness() that runs once after the Docker
daemon connection is established. It reads /proc/sys inotify/file-max
limits and /proc/filesystems (for FUSE), and logs a warning with the exact
commands to fix when they are too low for heavy containers -- XRd wants
~4000 inotify instances per node against a stock default of 128.

The server runs unprivileged (only the setuid ubridge helper has root), so
it can only check, not set; the warning tells the admin exactly what to
raise once. Stays silent when the limits are already sufficient.

tests/compute/docker/test_docker.py

@@ -364,3 +364,54 @@ async def test_install_busybox_no_executables():
                 dst_dir = Docker.resources_path()
                 await Docker.install_busybox(dst_dir)
             assert str(e.value) == "No busybox executable could be found, please install busybox (apt install busybox-static on Debian/Ubuntu) and make sure it is in your PATH"
+
+
+@pytest.mark.asyncio
+async def test_check_host_readiness_warns_when_low(caplog):
+
+    import logging
+    from io import StringIO
+
+    docker = Docker()
+    files = {
+        "/proc/sys/fs/inotify/max_user_instances": "128",
+        "/proc/sys/fs/inotify/max_user_watches": "8192",
+        "/proc/sys/fs/file-max": "100000",
+        "/proc/filesystems": "nodev ext4\nnodev tmpfs\n",
+    }
+
+    def fake_open(path, *args, **kwargs):
+        return StringIO(files[path])
+
+    with patch("builtins.open", side_effect=fake_open):
+        with caplog.at_level(logging.WARNING, logger="gns3server.compute.docker"):
+            docker._check_host_readiness()
+
+    message = " ".join(r.message for r in caplog.records)
+    assert "max_user_instances=128" in message
+    assert "sudo sysctl -w" in message
+    assert "modprobe fuse" in message  # FUSE missing -> warned
+
+
+@pytest.mark.asyncio
+async def test_check_host_readiness_silent_when_ok(caplog):
+
+    import logging
+    from io import StringIO
+
+    docker = Docker()
+    files = {
+        "/proc/sys/fs/inotify/max_user_instances": "64000",
+        "/proc/sys/fs/inotify/max_user_watches": "524288",
+        "/proc/sys/fs/file-max": "1000000",
+        "/proc/filesystems": "nodev ext4\nnodev fuse\n",
+    }
+
+    def fake_open(path, *args, **kwargs):
+        return StringIO(files[path])
+
+    with patch("builtins.open", side_effect=fake_open):
+        with caplog.at_level(logging.WARNING, logger="gns3server.compute.docker"):
+            docker._check_host_readiness()
+
+    assert not [r for r in caplog.records if r.levelno == logging.WARNING]