Docker build fails with 'no space left on device' (failed to solve ... rpc error) - how do I see what is using the disk and prune BuildKit cache safely?

Cause: the filesystem holding Docker's data (by default /var/lib/docker, or the Docker Desktop VM disk) is full. Images, stopped containers, volumes and BuildKit build cache all accumulate there, and BuildKit reports the failure as 'failed to solve' with an underlying write error. Unlike prune, the builder's own garbage collection only runs periodically and by its configured limits, so cache can still grow large between runs. Fix (best first): 1. Measure first: docker system df (add -v for per-image and per-volume detail) shows reclaimable space by type. Also check df -h on the data-root filesystem. 2. Prune only build cache: docker builder prune, or docker buildx prune. Narrow it with filters, for example --filter until=24h to keep recent cache, or use --max-used-space, --reserved-space or --min-free-space so the least recently used records are removed until a target is met. 3. Broader cleanup: docker system prune removes stopped containers, unused networks, dangling images and unused build cache; volumes are kept by default. Add -a to also remove all unused images, and --volumes only if you are sure no unused anonymous volume holds data you need; it can delete data permanently. Read the confirmation list before answering y. 4. Make it stick: configure BuildKit garbage collection, via builder.gc in daemon.json for the Docker driver (for example a defaultKeepStorage limit), or gcpolicy settings in buildkitd.toml for other drivers. 5. If the disk is simply too small, move data-root to a larger volume or, on Docker Desktop, raise the disk usage limit in Settings > Resources. Verify: docker system df shows reclaimed space, df -h shows free space on the Docker data filesystem, and the build completes. Sources: https://docs.docker.com/reference/cli/docker/system/prune/ https://docs.docker.com/reference/cli/docker/buildx/prune/ https://docs.docker.com/build/cache/garbage-collection/ https://docs.docker.com/reference/cli/docker/system/df/