ci: add GitHub Actions workflow to build Docker image

Triggers on pushes to server branch or version tags when
core/, server/, or Docker files change. Pushes to ghcr.io.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.github/workflows/docker.yml

@@ -0,0 +1,45 @@
+name: Docker Image
+
+on:
+  push:
+    branches: [server]
+    tags: ["v*"]
+    paths:
+      - "core/**"
+      - "server/**"
+      - "Dockerfile"
+      - "docker-compose.yml"
+      - ".github/workflows/docker.yml"
+  workflow_dispatch:
+
+permissions:
+  contents: read
+  packages: write
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: docker/login-action@v3
+        with:
+          registry: ghcr.io
+          username: ${{ github.actor }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+
+      - uses: docker/metadata-action@v5
+        id: meta
+        with:
+          images: ghcr.io/${{ github.repository }}-server
+          tags: |
+            type=ref,event=branch
+            type=semver,pattern={{version}}
+            type=sha,prefix=
+
+      - uses: docker/build-push-action@v6
+        with:
+          context: .
+          push: true
+          tags: ${{ steps.meta.outputs.tags }}
+          labels: ${{ steps.meta.outputs.labels }}