create build task

.github/workflows/build.yml

@@ -0,0 +1,25 @@
+name: build
+on: push
+jobs:
+  main:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - uses: actions/setup-node@v1
+        with: { node-version: 14 }
+      - id: yarn_cache
+        run: echo "::set-output name=dir::$(yarn cache dir)"
+      - uses: actions/cache@v1
+        with:
+          path: ${{ steps.yarn_cache.outputs.dir }}
+          key: yarn-${{ hashFiles('**/yarn.lock') }}
+          restore-keys: yarn-
+      - run: yarn
+      - run: yarn build
+      - name: The working tree is clean.
+        run: "[[ -z $(git status --porcelain) ]]"
+      - name: Show the working tree status.
+        if: failure()
+        run: |
+          git status
+          git diff