TopolearnImages and build cache quietly eat tens of gigabytes; knowing how to clean up keeps machines healthy.
Every image you pull or build stays on disk until you remove it, and so do stopped containers, unused volumes and the build cache.
Inspecting images:
docker image ls(ordocker images): repository, tag, ID, size.docker image ls --filter dangling=true: dangling images, untagged leftovers (<none>:<none>) from rebuilding a tag.docker image history nginx:1.27: the layers and the instructions that created them.docker tag nginx:1.27 myreg.example.com/nginx:prod: add another name. A tag is just a label; no data is copied.
Removing:
docker rmi nginx:1.27(docker image rm) removes a tag, and the image data once no tags or containers reference it. An image used by a container, even a stopped one, can't be removed without-f.docker image pruneremoves dangling images;-aremoves every image not used by a container.
The big picture: docker system df shows space used by images, containers, local volumes and build cache, and how much is reclaimable. docker system prune removes stopped containers, unused networks, dangling images and build cache. Add -a for all unused images, and --volumes to include unused volumes. Be careful with that last one: volumes hold data such as your local database.
docker builder prune clears just the build cache, often the largest item on a developer machine.
Example
docker image ls
docker image history --format '{{.Size}}\t{{.CreatedBy}}' python:3.13-slim | head -5
docker tag myapp:dev myapp:1.0.0 # second name, same image ID
docker system df
# TYPE TOTAL ACTIVE SIZE RECLAIMABLE
# Images 41 6 18.2GB 15.9GB (87%)
# Build Cache 312 0 9.4GB 9.4GB
docker image prune # dangling images only
docker builder prune --filter until=168h # build cache older than a week
docker system prune -a # everything unused (not volumes)Remember
- Tags are labels; one image can have many
- Dangling images are untagged leftovers from rebuilds
- docker system df shows where space goes; prune reclaims it
- Only prune volumes deliberately: they hold data
Watch out for
- Running docker system prune -a --volumes on a dev box and wiping your local databases.