CI: split up to several workflows

.github/workflows/build-image.yaml

@@ -0,0 +1,29 @@
+name: Build RPi image
+
+on:
+  push:
+    branches: [ '*' ]
+  pull_request:
+    branches: [ master ]
+  release:
+    types: [ created ]
+
+jobs:
+  build-image:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - name: Build image
+        run: |
+          docker run --privileged --rm -v /dev:/dev -v $(pwd):/builder/repo -e TRAVIS_TAG="${{ github.event.release.tag_name }}" sfalexrog/img-tool:qemu-update
+      - name: Compress image
+        run: |
+          sudo chmod -R 777 images && zip -9 $(echo images/clover_*).zip images/clover_* && ls -l images
+      - name: Upload image
+        uses: softprops/action-gh-release@v1
+        if: ${{ github.event_name == 'release' }}
+        with:
+          files: images/clover_*.zip
+          prerelease: true
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}