add github actions

Author: retroelec

.github/workflows/build.yml

@@ -0,0 +1,20 @@
+name: build c64win.exe
+on: [push]
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout Code
+        uses: actions/checkout@v4
+
+      - name: Build EXE with Docker
+        run: |
+          docker run --rm -v "${{ github.workspace }}":/build docker.io/retroelec42/sdl2-cross:latest make -f windows/Makefile
+
+      - name: Upload Executable
+        uses: actions/upload-artifact@v4
+        with:
+          name: windows-executable
+          path: "windows/c64win.exe"
+