buildctl reported: exporter "cacheonly" could not be found. Omitting --output
still runs every RUN step, which is all the probe needs. Same defect exists in
build-image-gitea.yml (validation path and toolchain assertion) and needs its
own fix.
The runner-pod probe cannot be transferred to buildkitd: different namespace,
different NetworkPolicies, so an UNREACHABLE there may be policy rather than
routing. This curls through a buildctl build, which egresses from the buildkitd
pod that actually has to push.