docker-compose.yml improvements

[zekroTJA]

Hey! I have some suggestions to make the setup with the provided docker-compose.yml a bit easier.

1. On a linux host, you can mount your /etc/machine-id to use persistent server auth with the /auth persistence Encrypted command. I've only added it as comment, because this will otherwise fail when the file does not exist on the system.
2. I've added the provided pre-built image to the file so that the server setup is a bit easier. If you don't like that, I can also remove this from the pull request if you want.

[Slowline]

Hey, thank you for your changes!
I'll merge your changes and add a new text block to the README mentioning the machine-id