added a nightly builds workflow

* this will both build and publish an AppImage and publish the LDoc-generated pages as the project's website (gh-pages)

.github/workflows/nightly.yml

@@ -0,0 +1,48 @@
+name: Nightly Builds
+
+on: [push, pull_request]
+
+jobs:
+  Linux:
+    # AppImages should be built on the oldest possible version.
+    # This should also match the version referenced in AppImage.yml.
+    runs-on: ubuntu-20.04
+
+    steps:
+
+    - name: Git Clone
+      uses: actions/[email protected]
+
+    - name: Update Repositories
+      run: sudo apt-get update
+
+    - name: Build AppImage
+      run: |
+        wget -O pkg2appimage.AppImage \
+                "https://github.com/AppImageCommunity/pkg2appimage/releases/download/continuous/pkg2appimage--x86_64.AppImage"
+        chmod +x pkg2appimage.AppImage
+        ./pkg2appimage.AppImage AppImage.yml
+        chmod a+x out/*.AppImage
+    - name: Archive AppImage
+      uses: pyTooling/Actions/[email protected]
+      with:
+        token: ${{ secrets.GITHUB_TOKEN }}
+        tag: nightly
+        files: out/*.AppImage
+
+    - name: Build Lua Documentation
+      run: |
+        # NOTE: The version from LuaRocks is just as old.
+        sudo apt-get install -y lua-ldoc make
+        make doc
+    - name: Publish Website
+      run: |
+        cd doc
+        touch .nojekyll
+        git init
+        cp ../.git/config ./.git/config
+        git add .
+        git config --local user.email "Website@GitHubActions"
+        git config --local user.name "GitHub Actions"
+        git commit -a -m "update ${{ github.sha }}"
+        git push -u origin +HEAD:gh-pages