Small docker fixes

[ConsoleCatzirl]

Add a demodata target to the docker entrypoint script, and update the documenation to use the entrypoint arguments.

[fajfer]

I just wanted to make exactly the same changes. Shame it wasn't reviewed for 1.5 year. @romcheg I know Ralph isn't super top priority thing in Allegro but this fixes both docs and docker setup. This fix would be very welcome