Initial implementation

[webbertakken]

Changes

• Add initial Dockerfile with script to run steamcmd and upload the build to steam.

Checklist

• [X] I read the contribution guide and accept the code of conduct.
• [x] Verify whether this works with a workflow
• [ ] Probably delete and start over with a better approach

[webbertakken]

This is close to a working example. I'm counting this is the initial implementation, although some parts may not fully work yet.