Use published Docker image for Binder

Noting that this issue was also raised in the binderhub repo:

I’m not sure it is fully solved by the repo2docker GitHub action. That solution still requires a “passthrough” one-liner binder Dockerfile (i.e. FROM myorg/myimage:latest). What @fortierq is asking for is the same thing I asked for in that issue: BinderHub directly launching a container from dockerhub, rather than its own private container inventory.

Also raised here: