feat: add docker build and push workflow

.github/workflows/docker.yml

@@ -0,0 +1,71 @@
+name: Docker Build & Push
+
+on:
+  push:
+    tags:
+      - 'v*'
+    branches:
+      - main
+    paths-ignore: ['**.md', 'docs/**', 'LICENSE']
+  workflow_dispatch:
+
+# Concurrency convention: see CONTRIBUTING.md → "GitHub Actions — Concurrency Convention".
+# Tag refs are unique per release — distinct tags run in parallel.
+# Pushes to main serialize; cancel superseded runs.
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: ${{ github.ref == 'refs/heads/main' }}
+
+jobs:
+  build-push:
+    name: Build & Push image
+    runs-on: ubuntu-latest
+    timeout-minutes: 30
+    permissions:
+      contents: read
+      packages: write
+
+    steps:
+      - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
+
+      # Required for multi-platform (linux/arm64) emulation.
+      - name: Set up QEMU
+        uses: docker/setup-qemu-action@29109295f81e9208d7d86ff1c6c12d2833863392 # v3.6.0
+
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@b5730b4f9e71e72c67a3b9d48e31e40de4b94f9d # v3.10.0
+
+      - name: Log in to GitHub Container Registry
+        uses: docker/login-action@74a5d142397b4f367a81961eba4e8cd7edddf772 # v3.4.0
+        with:
+          registry: ghcr.io
+          username: ${{ github.actor }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+
+      # Computes image tags and labels from Git metadata:
+      #   v*  tag  → ghcr.io/<owner>/<repo>:<semver>  (e.g. 1.2.3, 1.2, 1)
+      #   main push → ghcr.io/<owner>/<repo>:latest
+      - name: Extract Docker metadata
+        id: meta
+        uses: docker/metadata-action@902fa8ec7d6ecbea8a64d1257c2a3e5c84f5c28d # v5.7.0
+        with:
+          images: ghcr.io/${{ github.repository }}
+          tags: |
+            type=semver,pattern={{version}}
+            type=semver,pattern={{major}}.{{minor}}
+            type=semver,pattern={{major}}
+            type=raw,value=latest,enable={{is_default_branch}}
+            type=sha,prefix=sha-,format=short
+
+      - name: Build and push
+        uses: docker/build-push-action@14487ce63c7a62a4a324b0bfb37086795e31c6c1 # v6.16.0
+        with:
+          context: .
+          platforms: linux/amd64,linux/arm64
+          push: true
+          tags: ${{ steps.meta.outputs.tags }}
+          labels: ${{ steps.meta.outputs.labels }}
+          cache-from: type=gha
+          cache-to: type=gha,mode=max
+          build-args: |
+            BUILDPLATFORM=${{ runner.os == 'Linux' && 'linux/amd64' || 'linux/amd64' }}