Add Docker image documentation and workflow (#225)

Adds a CI action to build docker images and publish them to GHCR on
every commit. We can then use these when deploying the registry.

This turned out to be an upstream blocker of building the infra for
deployment, and something like this is needed for either deployment
approach we're exploring.

---

## Summary
- Add comprehensive documentation for pre-built Docker images in README
- Include usage examples and configuration guidance  
- Add GitHub Actions workflow for automated Docker image publishing

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-authored-by: Claude <noreply@anthropic.com>

.github/workflows/docker.yml

@@ -0,0 +1,47 @@
+name: Docker Build and Push
+
+on:
+  push:
+    branches: [ main ]
+
+jobs:
+  build-and-push:
+    runs-on: ubuntu-latest
+    permissions:
+      contents: read
+      packages: write
+
+    steps:
+    - name: Checkout repository
+      uses: actions/checkout@v4
+
+    - name: Set up Docker Buildx
+      uses: docker/setup-buildx-action@v3
+
+    - name: Log in to Container Registry
+      uses: docker/login-action@v3
+      with:
+        registry: ghcr.io
+        username: ${{ github.actor }}
+        password: ${{ secrets.GITHUB_TOKEN }}
+
+    - name: Extract metadata
+      id: meta
+      uses: docker/metadata-action@v5
+      with:
+        images: ghcr.io/${{ github.repository }}
+        tags: |
+          type=raw,value=latest,enable={{is_default_branch}}
+          type=sha,prefix=main-{{date 'YYYYMMDD'}}-,enable={{is_default_branch}}
+
+    - name: Build and push Docker image
+      uses: docker/build-push-action@v5
+      with:
+        context: .
+        file: ./Dockerfile
+        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