ci: add workflow to test dockerfile when it's edited

.github/workflows/test-dockerfile.yml

@@ -0,0 +1,19 @@
+ name: build Dockerfile
+
+on:
+  push:
+    paths:
+      - 'Dockerfile'
+  pull_request:
+    paths:
+      - 'Dockerfile'
+
+jobs:
+  build:
+   runs-on: ubuntu-latest
+
+   steps:
+     - uses: actions/checkout@v3
+     - name: Build Docker image 
+       run: docker build -t firo .
+