Add GHA config for formal checks

.github/workflows/formal.yml

@@ -0,0 +1,13 @@
+name: 'formal'
+
+on: [ push, pull_request ]
+
+jobs:
+
+  docker:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v1
+      with:
+        submodules: recursive
+    - run: docker run --rm -tv $(pwd):/src -w /src/formal -e BUILD_NAME=ACCEPTANCE ghdl/synth:formal make all