ci: deploy Docker image for every push on main branch

.github/workflows/docker.yaml

@@ -0,0 +1,36 @@
+name: push docker image
+
+on:
+  push:
+    branches:
+      - "main"
+
+jobs:
+  build_and_push:
+    name: build_and_push (${{ matrix.docker.repo }})
+    strategy:
+      matrix:
+        docker:
+          - repo: planetariumhq/9c-board
+    if: github.ref_type == 'branch'
+    runs-on: ubuntu-latest
+    steps:
+      - name: checkout
+        uses: actions/checkout@v4
+        with:
+          submodules: recursive
+      - name: login
+        run: |
+          docker login \
+            --username '${{ secrets.DOCKER_USERNAME }}' \
+            --password '${{ secrets.DOCKER_ACCESS_TOKEN }}'
+      - name: Set up QEMU
+        uses: docker/setup-qemu-action@v3
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v3
+      - name: build-and-push
+        run: |
+          docker buildx build . \
+            --platform linux/arm64/v8,linux/amd64 \
+            --tag ${{ matrix.docker.repo }}:git-${{ github.sha }} \
+            --push