Add Dockerfile.astra (astralinux/alse) and Dockerfile.redos (redos/redos)
for FSTEC-certified Russian OS targets. Update release.yml with a matrix
strategy that produces three image variants per release:
- ghcr.io/.../nora:0.x.x (Alpine, default)
- ghcr.io/.../nora:0.x.x-astra (Astra Linux SE)
- ghcr.io/.../nora:0.x.x-redos (RED OS)
Build stage is shared (musl static binary) across all variants.