workflow: add docker build and push job

.github/workflows/docker.yml

@@ -0,0 +1,45 @@
+name: docker
+
+on:
+  push:
+    branches: master
+
+jobs:
+  docker:
+    runs-on: ubuntu-18.04
+    steps:
+      - uses: actions/checkout@v2
+      - name: Install packages
+        run: |
+          sudo apt-get update
+          sudo apt-get install -y autoconf automake build-essential cmake curl file libtool
+      - name: Cross build amd64/arm64 binary
+        run: |
+          for arch in amd64 arm64; do
+            env BUILD_TARGET=$arch ./scripts/cross-build.sh
+            cp build/ttyd ttyd.$arch
+          done
+      - uses: docker/setup-qemu-action@v1
+      - uses: docker/setup-buildx-action@v1
+      - uses: docker/login-action@v1
+        with:
+          username: ${{ secrets.DOCKER_HUB_USER }}
+          password: ${{ secrets.DOCKER_HUB_TOKEN }}
+      - name: build/push docker image
+        uses: docker/build-push-action@v2
+        with:
+          context: .
+          file: ./Dockerfile
+          platforms: linux/amd64,linux/arm64
+          push: true
+          tags: |
+            tsl0922/ttyd:latest
+      - name: build/push docker image (alpine)
+        uses: docker/build-push-action@v2
+        with:
+          context: .
+          file: ./Dockerfile.alpine
+          platforms: linux/amd64,linux/arm64
+          push: true
+          tags: |
+            tsl0922/ttyd:alpine