Add CI workflow and configuration for SampSharp Docker image

[bssth]

I've created two versions - one with open.mp (which you can run directly on your code) and one without (which you can use as a scratch for your own image). Let me know if you don't like this approach

ikkentim/SampSharp#467

[ikkentim]

The user needs a Docker image that includes:

• the open.mp server
• the sampsharp component
• the .NET runtime

Ideally, we wouldn’t lock users into specific open.mp or .NET versions. However, open.mp releases are not as frequent as .NET, and obtaining an x64 build can be cumbersome. To simplify things, it’s preferable to provide an image containing open.mp and sampsharp, allowing users to layer in their preferred .NET runtime.

A typical Dockerfile for users could look like:

FROM <sampsharp> as sampsharp

FROM mcr.microsoft.com/dotnet/runtime:10.0 as runtime
# TODO: may need a few dependencies for open.mp?
COPY --from=sampsharp /server /server
COPY ./gamemode/ /server/gamemode
COPY ./config.json /server/config.json

# TODO: define ENTRYPOINT/CMD