Building your own (Domino) containers? Don’t forget to prune Docker

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If you regularly build or pull images on a Docker host, disk usage can creep up quickly.

Most people know that Docker (and Podman) keeps old images around unless you prune them. What’s less obvious is the build cache. When you build images, like the Domino container, Docker stores intermediate layers, so future builds can be faster.

Even if you build with --no-cache (as the Domino build script does), Docker still writes layers to the build cache. They’re just not reused. Over time, that cache can grow significantly.

In my case, it explained why my root disk was filling up: 25+ GB of build cache.

Housekeeping your Docker host

To see what Docker is using: docker system df

After building the Domino image twice, I saw:

TYPE            TOTAL     ACTIVE    SIZE      RECLAIMABLE
Images          39        35        23.44GB   5.132GB (21%)
Containers      37        37        817.8MB   0B (0%)
Local Volumes   119       11        1.412GB   14MB (0%)
Build Cache     12        0         4.909GB   723kb

A few things to note:

  • Images: reclaimable space is what you’d expect.
  • Build cache: can be large, and not always reclaimable at first glance.
  • For details, use: docker system df -v

Cleaning up

Basic cleanup options:

  • Remove unused (dangling) images: docker image prune
  • Remove all unused images (not tied to a container): docker image prune -a

The quick, all-in-one option: docker system prune

This removes:

  • stopped containers
  • unused networks
  • dangling images
  • unused build cache

A few gotchas

  • Tags vs. images:
    If your Domino container uses hclcom/domino:latest, then a tag like hclcom/domino:14.5.1IF1 may be removed by pruning. The underlying image stays, but the tag goes.
  • “Non-reclaimable” build cache:
    Cache linked to images still in use won’t be removed by default.
  • Force removal of all build cache:
    If you don’t care about reuse (e.g., with Domino builds), you can wipe everything: docker buildx prune -a

Conclusion

Docker doesn’t clean up after itself. If you build images regularly, especially with large builds like Domino, you’ll want to actively prune both images and build cache.

For most setups, a periodic:

docker system prune

is enough, but if you’re tight on disk space, it’s worth going a bit deeper.