add CI job for Docker Hub build and push

.github/workflows/docker.yml

@@ -0,0 +1,39 @@
+name: "Docker"
+
+on:
+  push:
+    branches:
+      - 'master'
+
+jobs:
+  docker:
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        variant: ["client", "server"]
+    steps:
+      -
+        name: Set up QEMU
+        uses: docker/setup-qemu-action@v2
+      -
+        name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v2
+      -
+        name: Login to Docker Hub
+        uses: docker/login-action@v2
+        with:
+          username: ${{ secrets.DOCKERHUB_USERNAME }}
+          password: ${{ secrets.DOCKERHUB_TOKEN }}
+      -
+        name: Build and push
+        uses: docker/build-push-action@v4
+        with:
+          file: ./Dockerfile.${{ matrix.variant }}
+          tags: m13253/dns-over-https-${{ matrix.variant }}:latest
+          push: true
+      -
+        name: Logout from Docker Hub
+        uses: docker/login-action@v2
+        if: always()
+        with:
+          logout: true