Add validate support via env

[gionn]

Hi,

sometimes it may be good to request a validate of existing data files:

-    STEAMCMD_ARGS+="+force_install_dir $STEAM_APP_DIR +app_update $STEAM_APP_ID "
+    STEAMCMD_ARGS+="+force_install_dir $STEAM_APP_DIR +app_update $STEAM_APP_ID validate "

it may be good to have an environment variable to control this behaviour when starting a new container.

[cliffrowley]

Good plan, thanks!