This repo contains the following (plain vanilla) Ubuntu Core Docker images:
- 15.10 / wily
- 16.04 / xenial
- 16.10 / yakkety
- latest (which is 16.10)
The images have been generated with the following command:
curl http://cdimage.ubuntu.com/ubuntu-base/releases/<version>/release/ubuntu-base-<version>-core-armhf.tar.gz | gunzip | sudo docker import - armv7/armhf-ubuntu_core:<version>
This images are quite small but not modified/optimized in any way for Docker usage. You may take a look at the 'official' ones (armv7/armhf-ubuntu), made on top of Ubuntu Core. If you expect snappy ubuntu here, you might be disappointed, it's the core part of Snappy Ubuntu Core only. I believe the snappy mechanism doesn't make much sense inside Docker containers, it's more for the outer world.
See here for more information about Docker on the ARMv7 platform.