add docker deploy step to github workflows (#1321)

Signed-off-by: Matthew Peveler <matt.peveler@gmail.com>

.github/workflows/dev_deploy.yml

@@ -33,6 +33,14 @@ jobs:
 
     - run: bundle exec middleman build
 
+    - name: Push to Docker Hub
+      uses: docker/build-push-action@v1
+      with:
+        username: ${{ secrets.DOCKER_USERNAME }}
+        password: ${{ secrets.DOCKER_ACCESS_KEY }}
+        repository: slatedocs/slate
+        tag_with_ref: true
+
     - name: Deploy
       uses: peaceiris/actions-gh-pages@v3.7.0-8
       with: