Add dev demo builds GitHub Actions workflow (#4980)

.github/workflows/demo.yaml

@@ -0,0 +1,39 @@
+name: Demo
+
+on:
+  push:
+    branches:
+      - dev
+jobs:
+  deploy:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Check out files from GitHub
+        uses: actions/checkout@v2
+      - name: Setting up Node.js
+        uses: actions/setup-node@v1
+        with:
+          node-version: 12.x
+      - name: Get yarn cache path
+        id: yarn-cache-dir-path
+        run: echo "::set-output name=dir::$(yarn cache dir)"
+      - name: Fetching Yarn cache
+        uses: actions/cache@v1
+        with:
+          path: ${{ steps.yarn-cache-dir-path.outputs.dir }}
+          key: ${{ runner.os }}-yarn-${{ hashFiles('**/yarn.lock') }}
+          restore-keys: |
+            ${{ runner.os }}-yarn-
+      - name: Install dependencies
+        run: yarn install
+        env:
+          CI: true
+      - name: Build Demo
+        run: ./node_modules/.bin/gulp build-demo
+      - name: Deploy to Netlify
+        uses: netlify/actions/cli@master
+        env:
+          NETLIFY_AUTH_TOKEN: ${{ secrets.NETLIFY_AUTH_TOKEN }}
+          NETLIFY_SITE_ID: ${{ secrets.NETLIFY_DEMO_DEV_SITE_ID }}
+        with:
+          args: deploy --dir=demo/dist --prod