Add 'release-docker' workflow

.github/workflows/release-docker.yml

@@ -0,0 +1,35 @@
+name: Release Docker Image
+
+on:
+  release:
+    types: [published]
+
+jobs:
+  build-and-push:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v4
+
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v3
+
+      - name: Log in to Docker Hub
+        uses: docker/login-action@v3
+        with:
+          username: ${{ secrets.DOCKERHUB_USERNAME }}
+          password: ${{ secrets.DOCKERHUB_TOKEN }}
+
+      - name: Extract version tag without 'v'
+        id: set_tag
+        run: echo "tag=${GITHUB_REF_NAME#v}" >> $GITHUB_OUTPUT
+
+      - name: Build and push Docker image
+        uses: docker/build-push-action@v5
+        with:
+          context: .
+          file: ./Dockerfile
+          push: true
+          tags: |
+            yhirose4dockerhub/cpp-httplib-server:latest
+            yhirose4dockerhub/cpp-httplib-server:${{ steps.set_tag.outputs.tag }}