add docker push to staging

Author: Southclaws

.github/workflows/stage.yml

@@ -25,3 +25,10 @@ jobs:
           args: release --rm-dist --snapshot
         env:
           GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+
+      - name: Publish
+        uses: elgohr/Publish-Docker-Github-Action@master
+        with:
+          name: picostack/pico
+          username: ${{ secrets.DOCKER_USERNAME }}
+          password: ${{ secrets.DOCKER_PASSWORD }}