add github action to compile the firewall

.github/actions/main.yml

@@ -0,0 +1,35 @@
+name: Main workflow
+
+on:
+  pull_request:
+  push:
+  schedule:
+    # Prime the caches every Monday
+    - cron: 0 1 * * MON
+
+jobs:
+  build:
+    strategy:
+      fail-fast: false
+      matrix:
+        os:
+          - ubuntu-latest
+        ocaml-compiler:
+          - 4.14.x
+
+    runs-on: ${{ matrix.os }}
+
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v2
+
+      - name: Use OCaml ${{ matrix.ocaml-compiler }}
+        uses: ocaml/setup-ocaml@v2
+        with:
+          ocaml-compiler: ${{ matrix.ocaml-compiler }}
+
+      - run: opam install --confirm-level=unsafe-yes "mirage>4"
+
+      - run: opam exec -- mirage configure -t xen && make depend && dune build
+
+      - run: sha256sum dist/qubes-firewall.xen