Add build workflow

.github/workflows/image.yml

@@ -0,0 +1,35 @@
+name: Build docker WordPress image
+
+on:
+  push:
+    branches: master
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - name: checkout code
+        uses: actions/checkout@v2
+      - name: Cache Docker layers
+        uses: actions/cache@v2
+        id: cache
+        with:
+          path: /tmp/.buildx-cache
+          key: ${{ runner.os }}-buildx-${{ github.sha }}
+          restore-keys: |
+            ${{ runner.os }}-buildx-        
+      - name: Set up QEMU
+        uses: docker/setup-qemu-action@v1
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v1
+        with:
+          config-inline: |
+            [worker.oci]
+              max-parallelism = 16
+      - name: Login to DockerHub
+        uses: docker/login-action@v1
+        with:
+          username: ${{ secrets.DOCKER_USERNAME }}
+          password: ${{ secrets.DOCKER_PASSWORD }}
+      - name: Build image
+        run: build/bin/build