新增 Gitea Actions 自動打包 Docker workflow

- 觸發：push 到 main、推送 v* tag、以及目標為 main 的 PR
- PR 事件只建置不推送（push=false），作為變更驗證
- 推 main：main + sha 標籤；推 v* tag：X.Y.Z / X.Y / X 版本標籤
- 登入 registry 使用 PUSH_PACKAGE_USERNAME / PUSH_PACKAGE_PASSWORD secrets
  （與 bear 倉庫 docker-publish 相同安排）
- buildx driver=docker（對接 runner 掛載的 daemon socket）

拆自 #3

.github/workflows/docker-publish.yml

@@ -0,0 +1,69 @@
+name: Docker Build & Push
+
+on:
+  push:
+    branches: [main]
+    tags: ["v*"]
+  pull_request:
+    branches: [main]
+
+env:
+  REGISTRY: gitea.alterminal.com
+  IMAGE_NAME: ${{ github.repository }}
+
+jobs:
+  build-and-push:
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      # The Gitea runner runs this job in a `node` container which has no
+      # Docker CLI. The docker daemon socket is mounted, so we only need the
+      # client (`docker login` / `docker` CLI) to authenticate and build.
+      - name: Install Docker CLI
+        run: |
+          apt-get update
+          apt-get install -y --no-install-recommends docker.io
+
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v3
+        with:
+          driver: docker
+
+      # Gitea Actions' automatic GITEA_TOKEN/GITHUB_TOKEN cannot publish to the
+      # container registry. A Personal Access Token is required instead:
+      #   - secrets.PUSH_PACKAGE_USERNAME: a user that can push to this org's packages
+      #   - secrets.PUSH_PACKAGE_PASSWORD: that user's PAT with write:package scope
+      # (Same arrangement as alterminal/bear .github/workflows/docker-publish.yml.)
+      - name: Log in to Gitea Container Registry
+        if: github.event_name != 'pull_request'
+        uses: docker/login-action@v3
+        with:
+          registry: ${{ env.REGISTRY }}
+          username: ${{ secrets.PUSH_PACKAGE_USERNAME }}
+          password: ${{ secrets.PUSH_PACKAGE_PASSWORD }}
+
+      # 本專案版本以 git tag（v0.0.1 …）為準：推送 v* tag 時產生
+      # X.Y.Z / X.Y / X 版本標籤；推送 main 另帶 branch 與 sha 標籤。
+      - name: Extract Docker metadata
+        id: meta
+        uses: docker/metadata-action@v5
+        with:
+          images: ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}
+          tags: |
+            type=semver,pattern={{version}}
+            type=semver,pattern={{major}}.{{minor}}
+            type=semver,pattern={{major}}
+            type=ref,event=branch
+            type=ref,event=pr
+            type=sha,format=long
+
+      - name: Build and push
+        uses: docker/build-push-action@v6
+        with:
+          context: .
+          push: ${{ github.event_name != 'pull_request' }}
+          tags: ${{ steps.meta.outputs.tags }}
+          labels: ${{ steps.meta.outputs.labels }}