Add a docker build helper for cleaning up untagged images.

docker/extras/README.md

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+# Docker build extras
+
+Optional utilities for builders.
+
+## docker-cleanup-dangling-images
+
+Daily cleanup of dangling (untagged) images. If you don't clean up old images you may eventually run out of disk space.
+
+First, copy the unit files into your build machine's `/etc/systemd/system/` folder.
+
+```bash
+cp docker-cleanup-dangling-images.{service,timer} /etc/systemd/system/
+```
+
+Alternatively `scp` can be much easier depending on your setup:
+
+```bash
+scp docker-cleanup-dangling-images.{service,timer} root@a.b.c.d:/etc/systemd/system
+```
+
+Now all you need to do is enable and start the timer.
+
+```bash
+systemctl enable --now docker-cleanup-dangling-images.timer
+```
+
+You can also trigger the cleanup job manually:
+
+```bash
+systemctl start docker-cleanup-dangling-images
+```