Improve management of docker image used for CI

[jcfr]

The image described in https://github.com/girder/large_image/blob/master/Dockerfile is currently published as zachmullen/tox-and-node and used in

large_image/.circleci/config.yml

Line 6 in ee805aa

- image: zachmullen/tox-and-node

I suggest to document how to rebuild the image and how to publish it on dockerhub.

Ideally, the image should be published in a project specific dockerhub organization.

Last, if this type of image is useful for other web effort, I wonder if should start agrageting them in common repository (a like what we do with dockcross) but that may be a stretch

Cc: @subdavis @mgrauer @zachmullen

[manthey]

Resolved with #429.