Add README with usage examples and size comparison

Document the Flutter SDK Docker images with:
- Image variants and sizes
- Comparison showing 75% smaller than alternatives
- CI/CD examples for GitHub, GitLab, and Gitea
- Security features (zero CVEs, SBOM, provenance)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

README.md

@@ -0,0 +1,107 @@
+# Flutter SDK Docker Images
+
+Minimal, secure Flutter SDK images for CI/CD pipelines. Built on [Wolfi](https://wolfi.dev) for a smaller footprint and zero CVEs.
+
+## Images
+
+| Variant | Use Case | Size |
+|---------|----------|------|
+| `svrnty/flutter-sdk:web-latest` | Web/WASM builds | ~800 MB |
+| `svrnty/flutter-sdk:android-latest` | Android APK/AAB builds | ~1.7 GB |
+| `svrnty/flutter-sdk:linux-latest` | Linux desktop builds | ~1.3 GB |
+
+### Size Comparison
+
+| Image | Size |
+|-------|------|
+| **svrnty/flutter-sdk:android** | **1.7 GB** |
+| cirrusci/flutter:stable | 4.5 GB |
+| ghcr.io/cirruslabs/flutter:stable | 6.8 GB |
+
+Up to **75% smaller** than alternatives.
+
+## Features
+
+- **Zero CVEs** - No critical, high, or medium vulnerabilities
+- **Multi-arch** - Supports both `linux/amd64` and `linux/arm64`
+- **Non-root** - Runs as unprivileged user (UID 65532)
+- **Minimal** - Only includes dependencies needed for each platform
+- **SBOM & Provenance** - Full supply chain attestations included
+
+## Usage
+
+### Web
+
+```bash
+docker run --rm -v $(pwd):/app -w /app svrnty/flutter-sdk:web-latest \
+  flutter build web
+```
+
+### Android
+
+```bash
+docker run --rm -v $(pwd):/app -w /app svrnty/flutter-sdk:android-latest \
+  flutter build apk
+```
+
+### Linux Desktop
+
+```bash
+docker run --rm -v $(pwd):/app -w /app svrnty/flutter-sdk:linux-latest \
+  flutter build linux
+```
+
+## CI/CD Examples
+
+### GitHub Actions
+
+```yaml
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    container:
+      image: svrnty/flutter-sdk:android-latest
+    steps:
+      - uses: actions/checkout@v4
+      - run: flutter build apk --release
+```
+
+### GitLab CI
+
+```yaml
+build:
+  image: svrnty/flutter-sdk:android-latest
+  script:
+    - flutter build apk --release
+```
+
+### Gitea Actions
+
+```yaml
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    container:
+      image: svrnty/flutter-sdk:android-latest
+    steps:
+      - uses: actions/checkout@v3
+      - run: flutter build apk --release
+```
+
+## Tags
+
+- `<variant>-latest` - Latest stable Flutter release
+- `<variant>-<version>` - Specific Flutter version (e.g., `android-3.38.9`)
+- `<variant>-dev` - Pre-release testing
+
+## Security
+
+All images are:
+- Scanned with Docker Scout on every release
+- Built with SBOM and SLSA provenance attestations
+- Based on Wolfi (minimal attack surface)
+- Automatically updated when new Flutter versions are released
+
+## License
+
+MIT