fix(docker): make x-ui CLI menu work inside containers

check_status() only recognized a systemd service or Alpine's
/etc/init.d/x-ui, neither of which exists in a container where the panel
runs as the foreground main process (PID 1 via "exec /app/x-ui"). Every
CLI command therefore failed with "Please install the panel first", and
restart/restart-xray relied on rc-service/systemctl that aren't present.

Detect the container (/.dockerenv or XUI_IN_DOCKER) and, when inside one:
- resolve the panel binary under /app instead of /usr/local/x-ui
- derive status from the running process instead of a service file
- restart via SIGHUP and restart-xray via SIGUSR1 to the panel process
- show Docker-appropriate guidance for start/stop/enable/disable

The Dockerfile sets XUI_IN_DOCKER/XUI_MAIN_FOLDER so detection is
explicit even though /.dockerenv alone suffices.

Closes #4817

Dockerfile

@@ -63,9 +63,9 @@ RUN chmod +x \
   /app/x-ui \
   /usr/bin/x-ui
 
+ENV XUI_IN_DOCKER="true"
+ENV XUI_MAIN_FOLDER="/app"
 ENV XUI_ENABLE_FAIL2BAN="true"
-# Database backend: set XUI_DB_TYPE=postgres and XUI_DB_DSN=postgres://... to use PostgreSQL.
-# Default (unset) is SQLite stored under /etc/x-ui.
 ENV XUI_DB_TYPE=""
 ENV XUI_DB_DSN=""
 EXPOSE 2053