The multi-stage Docker builder (which installed build-essential, compiled
soul-demo, and downloaded k3s inside Docker) was causing RWLayer nil
corruption on the runner's overlay2 driver. Every affected run failed at
apt-get install in the runtime stage after the builder stage completed.
Fix: move k3s download to the CI host runner (same pattern as soul-demo
compilation, which now passes reliably). Dockerfile.stage becomes single-
stage: no apt-get in a builder stage, no network downloads, just COPY of
pre-built binaries. Also adds --no-cache to the main docker build for
consistency with the soul-demo step fix.
will.anderson