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>

dockerfiles/sdk.Dockerfile

@@ -0,0 +1,10 @@
+FROM scratch
+ARG TARGETARCH
+ADD build-${TARGETARCH}/sdk/rootfs.tar.gz /
+COPY dotnet-${TARGETARCH}/sdk/ /usr/share/dotnet/
+ENV DOTNET_ROOT=/usr/share/dotnet
+ENV PATH="/usr/share/dotnet:${PATH}"
+ENV DOTNET_RUNNING_IN_CONTAINER=true
+ENV DOTNET_SYSTEM_GLOBALIZATION_INVARIANT=false
+WORKDIR /app
+USER 0