Docker: 'no space left on device' during docker build, pull or when a container writes. How do I find what's using the space and free it safely?

Cause: Docker stores images, containers, volumes, build cache and container logs under its data root (by default /var/lib/docker on Linux, or inside the Docker Desktop virtual disk on macOS and Windows). Old images, stopped containers, BuildKit cache and unrotated json-file logs pile up until that disk or virtual disk is full. Sometimes the disk still has free space but has run out of inodes, which gives the same error. How to check: - docker system df (add -v for detail) shows space used by images, containers, volumes and build cache, and how much is reclaimable. - df -h /var/lib/docker and df -i /var/lib/docker (inodes) on Linux. On Docker Desktop, check the disk usage limit under Settings, Resources. - Large logs: find the biggest *-json.log files under /var/lib/docker/containers. Fix (safest first): 1. docker builder prune removes build cache (add --all to remove all of it, not just unused). 2. docker image prune -a removes images not used by any container; docker container prune removes stopped containers. 3. docker system prune -a does all of the above at once. Volumes are only removed with --volumes, so check docker volume ls before adding it, because volumes hold databases and other data. 4. Stop runaway logs: set log rotation in /etc/docker/daemon.json, for example "log-driver": "json-file", "log-opts": {"max-size": "10m", "max-file": "3"}, then restart Docker. It applies to newly created containers. 5. If the disk is simply too small, increase the Docker Desktop disk limit or move the data root to a bigger disk with "data-root" in daemon.json. After: run docker system df again, and in CI add a scheduled prune (for example docker builder prune --filter until=72h) so build hosts don't fill up again. Sources: https://docs.docker.com/engine/manage-resources/pruning/ https://docs.docker.com/reference/cli/docker/system/df/ https://docs.docker.com/engine/logging/drivers/json-file/