Add Github Pages deployment

.github/workflows/deploy.yml

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+name: Deploy to Github Pages
+
+on:
+  push:
+    branches:
+      - main
+
+  workflow_dispatch:
+
+jobs:
+  deployment:
+    runs-on: ubuntu-latest
+
+    strategy:
+      matrix:
+        node-version: [14.x]
+
+    steps:
+      - uses: actions/checkout@v2
+
+      - name: Use Node.js ${{ matrix.node-version }}
+        uses: actions/setup-node@v1
+        with:
+          node-version: ${{ matrix.node-version }}
+
+      - name: Use NPM 7
+        run: npm i -g npm@latest
+
+      - name: Cache dependencies
+        uses: actions/cache@v2
+        with:
+          path: ~/.npm
+          key: ${{ runner.OS }}-node-${{ hashFiles('**/package-lock.json') }}
+          restore-keys: |
+            ${{ runner.OS }}-node-
+            ${{ runner.OS }}-
+      - name: Build
+        run: |
+          npm ci --legacy-peer-deps
+          npm run build
+          npm run export
+      - name: Deploy
+        uses: peaceiris/actions-gh-pages@v3
+        with:
+          deploy_key: ${{ secrets.ACTIONS_DEPLOY_KEY }}
+          publish_dir: ./out

env.config.js

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+const prod = process.env.NODE_ENV === 'production'
+
+module.exports = {
+  'process.env.BACKEND_URL': prod ? '/next-template' : '',
+}

next.config.js

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+const debug = process.env.NODE_ENV !== 'production'
+
+module.exports = {
+  assetPrefix: !debug ? '/next-template/' : '',
+}