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/build_test.go

@@ -30,7 +30,9 @@ func TestOutsideDirsSkipsSteamNoise(t *testing.T) {
 	r := &bootReport{ContainerChanges: []string{
 		"A /home/steam/.config/example-game/worlds/w.db",
 		"A /home/steam/Steam/logs/connection_log.txt",
+		"C /home/steam/Steam/config/config.vdf",
 		"C /home/steam/.steam/registry.vdf",
+		"A /usr/sbin/docker-init",
 	}}
 	got := outsideDirs(r)
 	if len(got) != 1 || got[0] != "/home/steam/.config/example-game/worlds" {