Minor tweaks

[glemsom]

• Use specific Ubuntu version (as per Dockerfile best practice)
• Use version variable in Dockerfile
• Rework required gamefiles
• Consolidate volumes
• Add run script to create default gamefiles

[Roemer]

Thank you for the PR. The change to one volume is breaking, so that should be noted in the readme somewhere. Also there are some non-generic strings around, especially in the run script :)
Can you update the PR, also fix the new conflicts?
Thanks

[Roemer]

Bump
Can you check my previous comment? I still would like to have that merged as I like the changes.