Docker "no space left on device" on a 200GB machine: the ghost volumes and dangling layers eating your disk

verbatim errorERROR: failed to solve: write /var/lib/docker/tmp/GetImageBlob: no space left on device # and in CI runners: failed to register layer: write /var/lib/docker/... : no space left on device

Problem

A build failed on a machine with 180GB free:

ERROR: failed to solve: write /var/lib/docker/tmp/GetImageBlob: no space left on device
# and in CI runners:
failed to register layer: write /var/lib/docker/... : no space left on device

"Free disk" according to Finder and df was plentiful. Docker thought otherwise, and Docker was right — about a filesystem the host cannot see.

Root cause

Docker's storage (/var/lib/docker, or the Docker Desktop VM's disk) is a bounded world of its own: images, dangling layers from failed/old builds, anonymous volumes (created by any VOLUME directive in an image — one per container run), build cache, and logs. Six months of dev on one machine routinely accumulates 50-100GB of dangling everything that no rm on the host will ever touch. macOS Docker Desktop is worse: the VM disk image grows but does not always shrink after cleanup, so even pruning inside Docker may not return space to the host.

fix preview — first 3 of 4 lines (bash), truncated:

docker system df
# TYPE      TOTAL   ACTIVE   SIZE     RECLAIMABLE
# Images    84      3        61.2GB   58.1GB (95%)
… 1 more line in the fix

🔒 the fix — including 3 code blocks — is members-only. $1/mo unlocks everything.