fix(builder): don't count Docker's init or the Steam client's own files as lost saves

Every container the node-agent starts gets /usr/sbin/docker-init, and SteamCMD
rewrites its own config.vdf on every run. Counted as files written outside the
volumes, they made every build look like it loses saves. A folder whose only
changes were logs no longer counts either, and a folder followed by a
same-named file (worlds, worlds.bak) is no longer taken for a file.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

cmd/gbrgames/firstboot.go

@@ -512,16 +512,36 @@ func (b *booter) fileLines(report *bootReport) []logLine {
 	return out
 }
 
-// System folders and files that aren't the game's state: what's written there doesn't matter when lost
+// Files that aren't the game's state, so losing them doesn't matter: system folders, Docker's own init
+// (in every container started with --init), logs, lock and pid files, and what the Steam client, Proton
+// and the OS keep for themselves in the home folder
 var (
-	systemPrefixes = []string{"/tmp", "/proc", "/sys", "/dev", "/run", "/etc", "/var/log", "/var/cache", "/var/tmp", "/var/lib/apt"}
+	systemPrefixes = []string{"/tmp", "/proc", "/sys", "/dev", "/run", "/etc", "/var/log", "/var/cache", "/var/tmp", "/var/lib/apt", "/usr/sbin/docker-init"}
 	transientExt   = map[string]bool{".log": true, ".pid": true, ".lock": true, ".tmp": true, ".swp": true}
+	clientNoise    = []string{"/Steam/logs/", "/Steam/config/", "/Steam/appcache/", "/.steam/", "/steamcmd/", "/.cache/", "/tmp/", "/.local/share/umu"}
 )
 
+func notGameState(p string) bool {
+	if transientExt[strings.ToLower(path.Ext(p))] {
+		return true
+	}
+	for _, prefix := range systemPrefixes {
+		if p == prefix || strings.HasPrefix(p, prefix+"/") {
+			return true
+		}
+	}
+	for _, n := range clientNoise {
+		if strings.Contains(p, n) {
+			return true
+		}
+	}
+	return false
+}
+
 // containerChanges lists the files the server wrote anywhere but /server and /data. All of them are
 // lost when the node recreates the container (every start): in the container's own filesystem, and in
-// the image's VOLUME folders, which Docker gives fresh anonymous volumes each time. Logs, lock and pid
-// files are left out; saves of any kind are not.
+// the image's VOLUME folders, which Docker gives fresh anonymous volumes each time. Files that aren't
+// the game's state are left out; saves of any kind are not.
 func (b *booter) containerChanges() []string {
 	var changes []string
 	if out, err := exec.Command("docker", "diff", b.container).Output(); err == nil {
@@ -541,7 +561,7 @@ func (b *booter) containerChanges() []string {
 		n := 0
 		for _, f := range strings.Split(strings.TrimSpace(string(files)), "\n") {
 			rel := strings.TrimPrefix(f, "/v/")
-			if f == "" || transientExt[strings.ToLower(path.Ext(rel))] {
+			if f == "" || notGameState(path.Join(dest, rel)) {
 				continue
 			}
 			if n++; n > 200 {
@@ -553,42 +573,42 @@ func (b *booter) containerChanges() []string {
 	return changes
 }
 
-// outsideFiles picks the files out of `docker diff` output: added or changed leaves (docker diff lists
-// folders too) outside the mounted volumes and system folders, without transient files.
+// outsideFiles picks the game's files out of `docker diff` output: added or changed outside the mounted
+// volumes. docker diff lists folders too, as changed when anything inside them changed, so a folder only
+// counts when nothing inside it did (it was created empty).
 func outsideFiles(diff string, mounts map[string]string) []string {
-	var paths []string
 	kinds := map[string]string{}
+	parents := map[string]bool{}
 	for _, line := range strings.Split(strings.TrimSpace(diff), "\n") {
 		kind, p, ok := strings.Cut(strings.TrimSpace(line), " ")
-		if !ok || kind == "D" || p == "" {
+		if !ok || p == "" {
 			continue
 		}
-		skip := transientExt[strings.ToLower(path.Ext(p))]
-		for _, prefix := range systemPrefixes {
-			if p == prefix || strings.HasPrefix(p, prefix+"/") {
-				skip = true
-			}
-		}
-		for mount := range mounts {
-			if p == mount || strings.HasPrefix(p, mount+"/") {
-				skip = true
-			}
-		}
-		if !skip {
-			paths = append(paths, p)
-			kinds[p] = kind
+		kinds[p] = kind
+		for d := path.Dir(p); d != "/" && d != "."; d = path.Dir(d) {
+			parents[d] = true
 		}
 	}
-	sort.Strings(paths)
-	var leaves []string
-	for i, p := range paths {
-		// A folder is followed (sorted) by the entries inside it
-		if i+1 < len(paths) && strings.HasPrefix(paths[i+1], p+"/") {
-			continue
+	var files []string
+	for p, kind := range kinds {
+		if kind != "D" && !parents[p] && !notGameState(p) && !inMounts(p, mounts) {
+			files = append(files, p)
 		}
-		leaves = append(leaves, kinds[p]+" "+p)
 	}
-	return leaves
+	sort.Strings(files)
+	for i, p := range files {
+		files[i] = kinds[p] + " " + p
+	}
+	return files
+}
+
+func inMounts(p string, mounts map[string]string) bool {
+	for mount := range mounts {
+		if p == mount || strings.HasPrefix(p, mount+"/") {
+			return true
+		}
+	}
+	return false
 }
 
 func (b *booter) cleanup() {