fix: Exclude TrueNAS Docker overlay mounts from disk stats

Host agent was including Docker overlay2 mounts from TrueNAS SCALE's
.ix-apps directory in disk totals. These mounts inherit the ZFS pool's
AVAIL space, causing massively inflated storage numbers (e.g., 173 TB
per container overlay instead of actual usage).

Changes:
- Add /mnt/.ix-apps/docker/ to container overlay path exclusions
- Use ShouldSkipFilesystem() in host agent disk collection (was only
  using ShouldIgnoreReadOnlyFilesystem() which missed container paths)
- Add test cases for TrueNAS overlay paths

Related to #718

internal/hostmetrics/collector.go

@@ -148,10 +148,12 @@ func collectDisks(ctx context.Context) []agentshost.Disk {
 			continue
 		}
 
-		// Skip read-only filesystems like squashfs (snap mounts), erofs, iso9660, etc.
-		// These are immutable and always report near-full usage, which causes false alerts.
-		// See issues #505 (Home Assistant OS) and #690 (Ubuntu snap mounts).
-		if fsfilters.ShouldIgnoreReadOnlyFilesystem(part.Fstype, usage.Total, usage.Used) {
+		// Skip filesystems that shouldn't be counted toward disk usage:
+		// - Read-only filesystems (squashfs, erofs, iso9660) - always report near-full
+		// - Virtual/pseudo filesystems (tmpfs, devtmpfs, cgroup, etc.)
+		// - Container overlay paths (Docker/Podman layers on ZFS, including TrueNAS .ix-apps)
+		// See issues #505, #690, #718, #790.
+		if shouldSkip, _ := fsfilters.ShouldSkipFilesystem(part.Fstype, part.Mountpoint, usage.Total, usage.Used); shouldSkip {
 			continue
 		}
 

pkg/fsfilters/filters.go

@@ -101,6 +101,12 @@ var containerOverlayPatterns = []string{
 	"/merged",
 }
 
+// containerPathPrefixes detect container-related paths that should be excluded
+// from disk usage, even if they don't contain "/containers/".
+var containerPathPrefixes = []string{
+	"/mnt/.ix-apps/docker/", // TrueNAS SCALE Docker overlay mounts
+}
+
 // ShouldSkipFilesystem determines if a filesystem should be excluded from disk
 // usage aggregation. It checks for read-only filesystems, virtual/pseudo filesystems,
 // network mounts, and special system mountpoints. Returns skip=true if the filesystem
@@ -155,5 +161,14 @@ func ShouldSkipFilesystem(fsType, mountpoint string, totalBytes, usedBytes uint6
 		}
 	}
 
+	// Check for container paths that don't follow the /containers/ pattern
+	// (e.g., TrueNAS SCALE uses /mnt/.ix-apps/docker/overlay2/...). Related to #718.
+	for _, prefix := range containerPathPrefixes {
+		if strings.HasPrefix(mountpoint, prefix) {
+			reasons = append(reasons, "container-overlay")
+			break
+		}
+	}
+
 	return len(reasons) > 0, reasons
 }

pkg/fsfilters/filters_test.go

@@ -180,6 +180,10 @@ func TestShouldSkipFilesystem(t *testing.T) {
 		{"custom containers diff", "ext4", "/opt/containers/myapp/diff/layer", 1000000, 500000, true},
 		{"custom containers overlay2", "ext4", "/data/containers/xyz/overlay2/layer1", 1000000, 500000, true},
 
+		// TrueNAS SCALE Docker overlay paths (issue #718)
+		{"truenas ix-apps docker overlay2", "zfs", "/mnt/.ix-apps/docker/overlay2/6c9aad59ec9b0bc33d5e374/merged", 173 * 1024 * 1024 * 1024 * 1024, 399 * 1024 * 1024, true},
+		{"truenas ix-apps docker overlay2 diff", "zfs", "/mnt/.ix-apps/docker/overlay2/c5e317c7340f9273d394dc0/diff", 173 * 1024 * 1024 * 1024 * 1024, 399 * 1024 * 1024, true},
+
 		// Windows paths
 		{"Windows System Reserved", "NTFS", "System Reserved", 500 * 1024 * 1024, 100 * 1024 * 1024, true},
 		{"Windows C drive - should NOT skip", "NTFS", "C:\\", 500 * 1024 * 1024 * 1024, 200 * 1024 * 1024 * 1024, false},