Here’s a small hack for squashing a Docker image to a single layer if you happen to run with /var/lib on a Btrfs filesystem. The output will be a tar file you can use with ADD in a Dockerfile to create a flat base image.

This is an alternative to docker create -ing a container and running docker export on it to get the filesystem contents. It saves you having to docker rm the container afterwards.

First, get the ID of the image you want to squash:

$ docker images rethink REPOSITORY TAG IMAGE ID CREATED VIRTUAL SIZE rethinkdb latest ffb24b60063b 6 days ago 181.8 MB rethinkdb 2.0.4 83d2da4505dc 2 weeks ago 195.8 MB

I’ll go with the latest version, ffb24b60063b . This is a prefix of the full image ID. The image will be a Btrfs snapshot in /var/lib/docker/Btrfs/subvolumes :

$ sudo -i # cd /var/lib/docker/btrfs/subvolumes # ls -d ffb24b60063b* ffb24b60063ba7e26ebf2f888deb5af0c8966dcbb612353fcd0c5d52a0a1d234

The Btrfs storage driver’s documentation goes into a fair bit of detail on how the driver uses Btrfs subvolumes and snapshots for images and containers.

Here’s an exerpt from dockviz output for that image that shows the various layers:

├─1565e86129b8 Virtual Size: 125.1 MB │ └─a604b236bcde Virtual Size: 125.1 MB Tags: debian:latest │ └─3012d15ee771 Virtual Size: 125.1 MB │ └─67ab878cc688 Virtual Size: 125.1 MB │ └─d4d9554f3430 Virtual Size: 125.1 MB │ └─d4bdd500e4ec Virtual Size: 125.1 MB │ └─9c56aa19b706 Virtual Size: 181.8 MB │ └─9102e5038f43 Virtual Size: 181.8 MB │ └─c5dff5ddf6a8 Virtual Size: 181.8 MB │ └─769806cae856 Virtual Size: 181.8 MB │ └─ffb24b60063b Virtual Size: 181.8 MB Tags: rethinkdb:latest

Then it’s just a matter of:

$ sudo -i # cd /var/lib/docker/btrfs/subvolumes/ffb24b60063ba7e26ebf2f888deb5af0c8966dcbb612353fcd0c5d52a0a1d234 # tar c . > /tmp/rethink-squashed.tar

Tab completion should pick up the full directory name from the short image ID, so you don’t have to copy the complete ID.

That’s pretty much it. You can get a flat base image with a Dockerfile with these contents: