ci: test building Docker image on push

.github/workflows/ci.yml

@@ -30,3 +30,12 @@ jobs:
     - name: Lint and Test
       run: |
         bundle exec rake ci
+
+    # https://github.com/marketplace/actions/build-and-push-docker-images
+    - name: Set up Docker Buildx
+      uses: docker/setup-buildx-action@v1
+
+    - name: Test building Docker image for dyndnsd
+      uses: docker/build-push-action@v2
+      with:
+        context: docker