Initial commit: multi-version .NET Docker images with apko base

- apko configs for runtime (ICU), runtime-invariant (no ICU), and SDK variants
- Build workflow with dynamic matrix from .NET release metadata (EOL-aware)
- Daily update-check workflow to detect new .NET versions
- Docker Scout PR analysis workflow
- LTS/STS floating tags, multi-arch (amd64 + arm64)
- Makefile for local builds and version discovery

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

.gitignore

@@ -0,0 +1,5 @@
+build-*/
+dotnet-*/
+context-*/
+*.tar.gz
+.env