Intially you have to create or load a game once after first startup. After that the Autoload Mod jump starts into the previously loaded savegame everytime you rerun the container. You can also edit the config file of the Autoload Mod to archieve similar behaviour.
The game, the modloader (SMAPI), and the "Always On" Mod is pulled from my servers to minimize version conflicts. Afterwards everything will be unpacked into the right places. In the end of the init script the game will be started inside Xvfb.