Support pushing to multiple registries

[yuvipanda]

Is your feature request related to a problem? Please describe.

When used with JupyterHubs, you often want to push the built image to two registries - a private one (like AWS ECR) closer to your JupyterHub, and a public one (like DockerHub or quay.io) for use with mybinder.org / others.

Describe the solution you'd like

Some way to specify additional tags the image should have, multiple forms of authentication, and an indicator that they should all be pushed.

Describe alternatives you've considered

This can be currently done with an extra step in the github workflow, doing a docker tag and a docker push.