Docker image format migration tool for Arvados.

This converts Docker images stored in Arvados from image format v1
(Docker <= 1.9) to image format v2 (Docker >= 1.10).

Requires Docker running on the local host.

Usage:

1) Run arvados/docker/migrate-docker19/build.sh to create
arvados/migrate-docker19 Docker image.

2) Set ARVADOS_API_HOST and ARVADOS_API_TOKEN to the cluster you want to migrate.

3) Run arv-migrate-docker19

This will query Arvados for v1 format Docker images.  For each image that does
not already have a corresponding v2 format image (as indicated by a
docker_image_migration tag) it will perform the following process:

i) download the image from Arvados
ii) load it into Docker
iii) update the Docker version, which updates the image
iv) save the v2 format image and upload to Arvados
v) create a migration link