forgejo-runner labels nullstone -> fedora:43 image. Switching
runs-on: ubuntu-24.04 -> nullstone makes the job container itself
the build environment, eliminating the docker-in-docker workspace
bind-mount problem (host path != act-container path).
Build now runs as root in fedora:43, installs livecd-tools directly
via dnf, and writes outputs to $GITHUB_WORKSPACE which is the natural
runner workdir on host. No nested docker, no userns juggling, no
explicit -v workspace bind needed.
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