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>

apko/runtime-invariant.yaml

@@ -0,0 +1,30 @@
+contents:
+  repositories:
+    - https://packages.wolfi.dev/os
+  keyring:
+    - https://packages.wolfi.dev/os/wolfi-signing.rsa.pub
+  packages:
+    - wolfi-baselayout
+    - glibc
+    - libstdc++
+    - ca-certificates-bundle
+    - libssl3
+    - zlib
+
+accounts:
+  groups:
+    - groupname: nonroot
+      gid: 65532
+  users:
+    - username: nonroot
+      uid: 65532
+      gid: 65532
+  run-as: 65532
+
+archs:
+  - x86_64
+  - aarch64
+
+environment:
+  DOTNET_RUNNING_IN_CONTAINER: "true"
+  DOTNET_SYSTEM_GLOBALIZATION_INVARIANT: "true"