Use repos/releng/buildkit:wmf-v0.11-1 image for buildkit
Use docker-registry.wikimedia.org/repos/releng/buildkit:wmf-v0.11-1 instead of docker-registry.wikimedia.org/releng/buildctl:0.10.4-4.2 to acquire the buildctl executable.
While in here, advance to the latest bullseye base image for kokkuri.