Add AppImage build

[andy5995]

@Jammyjamjamman You said that you had problems when building the appimage locally when you try it in the docker build-env container? What kind of problems or errors are you having?

[andy5995]

Works great!

It would be super if you could add the git version to the filenames.

You can use git rev-parse --short HEAD

Usually if the VERSION env var is set, the AppImage is created with that as part of the filename automatically.

What you'll probably want to do for convenience is near the top of the CI workflow (after checkout), something like

run: |
        echo "VERSION=$(git rev-parse --short HEAD)" >> $GITHUB_ENV

Then you can use VERSION in all the following parts of the workflow, and pass it to any external scripts. In other contexts, you'll need ${{ env.VERSION }} but I think you already know when to use that syntax...

[Jammyjamjamman]

Added all requests, I think.

Unfortunately, git commit hash doesn't seem to work on appimage build. See "Git Hash": https://github.com/Jammyjamjamman/megaglest-source/actions/runs/4400454984/jobs/7705765874

(Looks like an access problem with $GITHUB_ENV in containers.)