A simple docker container and image garbage collection script.

• Containers that exited more than an hour ago are removed.
• Images that don't belong to any remaining container after that are removed.

Although docker normally prevents removal of images that are in use by containers, we take extra care to not remove any image tags (e.g. ubuntu:14.04, busybox, etc) that are in use by containers. A naive docker rmi $(docker images -q) will leave images stripped of all tags, forcing docker to re-pull the repositories when starting new containers even though the images themselves are still on disk.

This script is intended to be run as a cron job.

Build the debian package:

$ apt-get install git devscripts
$ git clone https://github.com/spotify/docker-gc.git
$ cd docker-gc
$ debuild -us -uc -b

$ dpkg -i ../docker-gc_0.0.1_all.deb

This installs the docker-gc script into /usr/sbin and drops a docker-gc cron file into /etc/cron.hourly/.

To use the script manually, run docker-gc.

There can be images that are large that serve as a common base for many application containers, and as such, make sense to pin to the machine, as many derivative containers will use it. This can save time in pulling those kinds of images. There may be other reasons to exclude images from garbage collection. To do so, create /etc/docker-gc-exclude, or if you want the file to be read from elsewhere, set the EXCLUDE_FROM_GC environment variable to its location. This file can contain image name patterns (in the grep sense), one per line, such as spotify/cassandra:latest or it can contain image ids (truncated to the length shown in docker images which is 12.

An example excludes file might contain:

spotify/cassandra:latest
9681260c3ad5