Fix Steam login flicker with 2GB shm and stable launch scripts.

Use tini-backed entrypoint, two-phase Steam startup, and a window keeper so CEF UI and the fluxbox toolbar stay visible in Moonlight.

Co-authored-by: Cursor <cursoragent@cursor.com>

docker-compose.yml

@@ -9,6 +9,7 @@ x-game-template: &game-template
     - seccomp:unconfined
     - apparmor:unconfined
   ipc: shareable
+  shm_size: "2gb"
   environment:
     NVIDIA_DRIVER_CAPABILITIES: all
     DISPLAY: ":99"