ci: use rake to build Docker image

.github/workflows/ci.yml

@@ -31,13 +31,3 @@ 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: .
-        file: docker/ci/Dockerfile