chore(ci): add simple build workflow

[dnwe]

Add a simple GitHub Action workflow which builds (and caches) the .devcontainer docker image and then runs a make of the MiSTer binary within it before uploading it as an artifact on the workflow result.

This provides a simple compilation test on pull requests and also allows the author or others to grab a built binary for testing purposes

[sorgelig]

It's not a good idea to make a binary with every change in this repo.
There is unstable/nightly builds which is fine for those who want to test latest changes.

[dnwe]

To be clear, the binary is only attached to the workflow result, it isn’t added to the repo or anything. It is mostly useful as confirmation that when someone has raised a PR their changes do compile. However, a side benefit is (for example) if someone submits a pull request to add support for a new steering wheel in input.cpp there’s also a binary available, build in a clean env, that can be linked to owner(s) of that wheel to test and confirm the changes are good