Each build launches a plain images:debian/12 container and apt-installs runc + downloads buildkit before it can build (~1-2 min of bootstrap per build). Ship a prebuilt hydra-builder OCI image (debian + runc + buildkitd/buildctl + git, buildkitd ready) to scaleregistry, and default builder.image to it, so the ephemeral builder is ready immediately. The dispatch bootstrap becomes a no-op when the tools are already present (it already guards with command -v buildkitd). Note: for the future release/AppImage build type (#523) this image also carries the Qt/AppImage deps. Related: #508, #523.