Configure BuildKit garbage collection policies in buildkitd.toml for a custom Docker builder

Documented steps

1. Create a docker-container or kubernetes builder that honors BuildKit config: docker buildx create --name ci --driver docker-container --use (buildkitd.toml configuration is only honored by non-docker drivers; the default docker driver reads daemon.json instead).
2. Enable GC and set disk thresholds under the worker section, e.g. [worker.oci] gc = true, reservedSpace = "10GB", maxUsedSpace = "100GB", minFreeSpace = "20%" — values accept plain bytes, unit strings like 512MB, or percentages of total disk (reservedSpace default: 10% of disk or 10GB whichever lower; maxUsedSpace default: 60% of disk or 100GB; minFreeSpace default: 20GB).
3. Add ordered GC policies as [[worker.oci.gcpolicy]] blocks; policies are evaluated in sequence, most specific first, and broader rules only run if earlier ones don't reclaim enough.
4. Policy 1 example drops easily-regenerated cache: filters = ["type==source.local", "type==exec.cachemount", "type==source.git.checkout"], keepDuration = "48h", maxUsedSpace = "512MB".
5. Policy 2 example drops cache unused 60+ days: keepDuration = "1440h", reservedSpace = "10GB".
6. Restart the builder so the config takes effect (docker buildx stop then docker buildx create/inspect --bootstrap, or recreate the builder since containerized buildkitd reads config at startup).
7. Official docs: https://docs.docker.com/build/cache/garbage-collection/ ; https://docs.docker.com/build/builders/drivers/